I was just wondering about that as well, whether college groups whose membership had changed could re-enter. I personally would have no problem with it.
Why do you think so many new groups formed for the show? Is there a shortage of acapella groups that are really talented? I don't pay much attention to the genre outside of this show, so I don't really know. I would guess that there are many groups doing it, but not a whole lot who are at the top of their game. But maybe I'm wrong.