Two possible reasons. Reason one: Either Buffy hadn't, at the time of Summer's gang's invasion of the dorm room,, developed any sort of personal feelings of it as her new home (remember, ALL of Sunday's victims seemed to be freshmen, just arrived on campus, somewhat disoriented and feeling pretty much lost. It's doubtful that any of them had begun feeling that their dorm rooms as any kind of metaphor for "home," therefore the vampire/threshold injunction wouldn't be in effect) whereas, later, once she was fully oriented with her new life and settled in, the room became "home" to her, therefore Angel required an invitation.
Second possible reason: The writers either screwed up or intentionally violated vampire protocol for the sake of the story, either not thinking that the viewers would notice or hoping that they just wouldn't care. (Obviously, the writers don't know Buffy fans the way that they should. We see everything, we miss nothing, and we do more analysis of facts than Sherlock Holmes when the game's afoot.)