I was thinking about these lines from The Trial:

Angel:  "I may not be able to come in, Lindsey, but..."
Lindsey, taking another sip:  "Wipe your feet."
Angel:  "What?"
Lindsey gets up:  "Wipe your feet.  You can come in."

Do you think the first "Wipe your feet" did it?  How implied or concrete does an invitation need to be?

I will now refrain from posting stupid "I don't know, but I think" info on the forum  - sorry for any stupid info I posted -

nmcil, I'm not sure what you're talking about in canon vs. fanon on invites, but "I never disinvited you" would be a reference to the de-invite spell that Willow did - they used it on Angelus in season 2, and then Dracula, Harmony, and Spike in season 5. Once a vampire is invited into a house, he or she is able to enter freely from that point on, unless the de-invite spell is done.

Protection spells (or more accurately, barrier spells, like the one Willow did in "Spiral" and the magical barrier in "Showtime" ) are totally separate from vampire invitations. The one in "Spiral" presumably kept everyone out, since a hole had to be made for anyone to pass. It's unclear whether the barrier in "Showtime" was specific to the Turok-han or not, but I would guess it was similar to the other one.

Protection spells, as defined on the show, don't seem to go around houses, but rather protect specific people (like Anya's protection spell for Jonathan and Andrew in "Two To Go" ). I don't believe we've ever seen the type of protection spells that show up in fics, like where any demon who tries to enter trips an alarm or meets a barrier.

As for the original question, the invitation doesn't seem to need to be very specific. For example, Angel is able to enter Cordy's apartment because she told him he was "welcome any time" (while at Angel's office, not at her door), and is also able to enter a client's house because she said to "stop by." I thought there was an instance where Angel asked for an invitation, and the person was somewhat vague and Angel said, "I need a little more than that," or something, but I could be remembering a fic.

It also doesn't have to be spoken word--Angelus gets into the school because of the sign out front--"All may enter who seek knowledge" or something.  I assume that Spike and his vamps got in because of the same sign, or because of Parent/Teacher night, where there was a big "Welcome" banner.  I assume because of those, anyone with an unfortunate "Welcome "doormat is asking for all kinds of trouble.

As for tripping alarms and what have you, didn't Willow and Tara kind of set something like that up in season five for Glory?

ETA:  I think Angel does say that in some ep, E.  Don't recall which one though

Oh, that reminds me, also in Guise Will Be Guise, the swami says, "Well, in or out?  You're letting in the bugs."  And in Eternity, that actress does say, "Well, stop by.  I’ll give you a private screening of the episode I didn’t win the Emmy for," which is apparently all Angel needs to crash through her window and save her from the fake stalker/assassin.  'Stop by' is the most informal invitation I think we've heard on the show.  Hmm...

The school is a public place. Vampires stroll in and out all the time - Angel, right from season I, Spike and crew, Drusilla, etc.  I'm pretty sure it's only private homes/apartments/etc. that are protected. Apparently dorm rooms, although we don't know if Spike actually needed Willow's "come in", but it was certainly implied that he did.  Don't know about other temporary abodes, like hotel rooms or cabins in the woods.

Yeah, the dis-invite is a spell specifically to ban a particular vampire that had already been given access.

I'm not sure how consistent they were in canon as to the actual words.   Mostly, though, it seems as though someone has to actually say "come in".

E - Cordelia could have said the words for Angel at some other time, like when she got home.  She could have remembered telling him he was welcome and just said "come in, Angel" when she got home. That's assuming he doesn't have to actually be standing there.  Don't know about the client. Could just be one of those brain farts on the part of the writers; or maybe the "why don't you stop in?"  might be all it takes.  I'm voting for brain fart.

Hotel rooms and cabins etc. don't count--Angel busts right into Faith's motel room to save Xander. 

As for the school, well, Angelus makes mention of it, so I assumed it was important.  And Jenny's all like "how did you get in?"  Which could be considered strange, since Darla is shown in the school, Spike and his vamps, and I'm sure others.  But I assume the sign has been up there all that time.  How could Jenny or Giles not notice that?  :shrug:  The sign's in Latin, but I assume they boths speak or read it.

Hotel rooms they don't need an invite for.  Angel says that to Holtz, and he enters someone else's hotel room with Gunn, I think.

Edit:  Scarlet beat me to it, LOL.

I think the writers just goofed on the school thing.  There was no reason to explain how Angelus got in, or why Jenny should ask how he got in.  Vamps go to public places without invites all the time.  The hospital, the Bronze, the magic shop...

Though it's a public school, if you're not a staff member or a student, you do need a pass (permission) for entry, right?  Or at least you did when I went to school.  Anyway, it's the only explanation I could come up with, and differentiates a slight difference from a school, and hospitals, clubs, shops, etc.

I didn't watch AtS as closely as I watched BtVS, but there is some episode where a flashback of Angelus and Darla at Holtz's house and the little girl opens the door. They ask to come in, and she moves away from the door, and Angelus asks if that's a yes, then? She nods. That lets in both Angelus and Darla. In BtVS Real Me ep, Dawn invites Harmony in by accident, and her minions can't enter and Xander says the invite is only good for one. I think this is season four of Angel and five of Buffy, so it's not like the writers were getting the hang of the rules anymore, so it kind of bugs me. In earlier seasons I can forgive the inconsistencies more, but then they just get lazy depending on what they need for that one scene!

As for the school, well, Angelus makes mention of it, so I assumed it was important. And Jenny's all like "how did you get in?" Which could be considered strange, since Darla is shown in the school, Spike and his vamps, and I'm sure others. But I assume the sign has been up there all that time.

my husband brought up a good point. maybe the question of "how did you get in?" had nothing to do with how he got an invite, but how he physicaly got in. most schools i know are locked up after hours. jenny ,seemingly, was the only one there that night, so theoreticly, the school should have been locked up.
but as many discutions about canon, all the invitation business is screwy and inconsistant.

As for tripping alarms and what have you, didn't Willow and Tara kind of set something like that up in season five for Glory?
You're right, they did. I forgot about that one. I'm guessing that only worked on Glory, since Willow says it's for something "hellgodishly powerful," and Spike didn't set it off when he walked in the house in the next episode. Also, if it was multi-purpose, you'd think they'd have used it more often (like in season 7).

Cordelia could have said the words for Angel at some other time, like when she got home. She could have remembered telling him he was welcome and just said "come in, Angel" when she got home.
Nope, she didn't. She's surprised when Angel just walks in, and says, "What about the rule?" He says, “You said when you got a place I was completely invited over.” So, apparently, Cordelia didn't even need to have the apartment at the time when she invited him in...

Apparently dorm rooms, although we don't know if Spike actually needed Willow's "come in", but it was certainly implied that he did.
It's inconsistent. Angel claims he needs an invite to enter Buffy's room, but Sunday and her gang were obviously breaking into people's rooms, and they didn't need invitations, so I think Angel must have been confused...

But OOH! I just found the quote I was talking about before. It's in "The Yoko Factor":

Angel:  Hi.  Can I come in?
Buffy: (softly) I guess.
He hesitates.
Angel:  Uh, I need a little more than that.
Buffy:  Oh.  Um . . . come in.

He's not just being polite, since "I guess" would cover it, but I don't see how Sunday and her gang could've gotten in unless they didn't need an invitation.

I think the writers just goofed on the school thing. There was no reason to explain how Angelus got in, or why Jenny should ask how he got in.
I agree. You can wank it all you want, but a school is just as much a public building as a hotel or a store (even more so, since a public high school is owned by the city, whereas a hotel or store would be owned by an individual), so there's no reason to need an invitation.

maybe the question of "how did you get in?" had nothing to do with how he got an invite, but how he physicaly got in.
Maybe that was her question, but then his answer should've been, "Uh, I broke the lock. I have superstrength, remember?" not his bullshit about the Latin.

Just reread the transcript from "The Freshman," and I have some additional thoughts on the dorm invite thing.

It seems Sunday kills Eddie (Buffy's lost buddy) and then the gang goes to the room to take his stuff, which works since Eddie is in a single room. He's dead, thus no invite needed. However, how many freshmen get single rooms? And judging by the amount of stuff they have, they've obviously pulled this trick several times. So, unless they killed both roommates, they wouldn't be able to get in. It's unlikely that they'd ask the roommate for an invite, since they're there to steal the victim's stuff and make it look like he or she packed up and left college. It's also unlikely they'd try to set it up that both roommates left, because what are the chances of that happening?

All that aside, even if they only targeted people with single rooms or worked out some other plan to get in, there's no way for them to get into Buffy's dorm without Buffy inviting them in. Buffy's obviously not dead, and she didn't issue an invite, yet they were able to break in and steal her stuff. Kathy can't issue an invite, because she's a demon, and the rule only works for humans. Also, when Buffy goes missing, Kathy doesn't mention anything about a group of people stopping by, claiming to be friends of Buffy's, so we can assume she never saw them.

It seems the magic of the door barrier seems to partly depend on what the vampire believes is true, as well as what the human said. If the vampire interprets the intent to be enough of an invite, then they can come in. It's the only real consistency in the varying ways of entry on the shows.

At my college, you only needed to get past the front door of the dorm to be able to wander all over it. Most people had their doors wide open unless they were changing, sleeping, or not there. Everybody treated it as more like bedrooms in a house, then apartments with full privacy, especially because there was a shared bathroom for every 2 rooms. Honestly, I always would have thought of dorms in relation to vampires the same as a house - they only need to get past the front door. Students don't really own anything in those buildings.

CM

Hotels vs. Dorm Rooms: I believe that Angel explained that he doesn't need an invitation to a hotel room as it is not a residence.  It was in some episode where he breaks into a hotel room with Gunn, but I don't all of the AtS episodes by heart.  The dorm room, however, is generally considered a residence, if only a temporary one.  People staying in hotels aren't usually there with the intention to reside.  Although I suppose that people actually living in hotel rooms permanently, who intend to reside there, would need to invite in a vampire.

Sunday and her posse: I don't know about everyone else, but when I was in college, strangers were always buddying up with each other, especially freshmen.  Most likely, Sunday and her peeps hung out with the college students, went to parties, trolled the dorms.  The college environment easily lends itself to complete strangers going in and out of each other's dorm rooms, houses, apartments; getting to know new people; hanging out in large groups: "Dude, there's a party in my room!"   People can meet at a bus stop and are suddenly inseparable.  Some guys and girls hook up with nameless bodies on a regular basis.  It's completely possible for Sunday and her gang to get invited into dorm rooms all the time.  Not to mention that, if they turned someone, that familiar person could visit all of his or her friends and get invited in just like we see the vamps on the shows kill their families after newly rising.  It's definitely not far fetched.

School: Simply put, the writers f-ed up.  Joss may be God, but he's not infallible :-P

"Oh, that reminds me, also in Guise Will Be Guise, the swami says, "Well, in or out?  You're letting in the bugs." 

Hope I'm not spoiling anybody, but Guise will be Guise is not a good example of the invite problem because the guy standing in the doorway is not the real swami, but a hit man who already killed the swami and dumped his body.

He probably needed to make it look like he invited Angel, but Angel could have come in at any time.

Varin

Oops.  Yeah, you're right.  Real swami was already dead.

It's completely possible for Sunday and her gang to get invited into dorm rooms all the time.
Other people's maybe, but Buffy's room? Buffy obviously didn't invite her in, and Kathy can't. I could buy that Sunday and her posse only needed an invitation to get into the building, not the individual dorm room - but the problem is that conflicts with what Angel says about needing an invite into Buffy's room. Which I guess would be another case of "the writers f-ed up."

Maybe Angel just wanted to hear her give a definitive answer to him being allowed to come in besides, "I guess."  That response is close along the lines of "whatever," and they were kind of on the rocky at that point, right?  I blocked out about half of s4, but he knew about Riley then, or this was post Faith.  I wouldn't put it past Angel to just wanting to hear the invite as opposed to actually needing one :P

I just watched "Reunion," and after Dru and Darla crash the tasting party, Holland's wife says, "Help us," to Angel, which seems to let him enter.