I'm not sure about how all groups were divided, but my group was accelerated only (and a few of us accelerated also okay with being considered for traditional if not accelerated). The groups are predetermined before the interview starts.
I'm not totally sure, but I think you are scored for your essay, GPA (and the science pre-reqs), as well as your interview, then they evaluate your total score. I'm sure a lot of the point of the interview is too see if you are polite, enthusiastic about the program (really intend to attend if accepted/have done a little homework), and give an impression that you can commit to and succeed in the program. Also perhaps to see if anyone is rude, aggressive (speaking over others/not listening to others or what's being asked) or seem uninterested despite high GPA's and well-written essays.
My group had only four students, not five like they said in the email. Also, the interview was almost the last part of the day, not right after check-in like my email said. The interviewers were friendly. The day was interesting and almost over by noon. There weren't very many questions, and none too "weird" to throw you off and no solving problems as a group to demonstrate teamwork or anything like that (which I was afraid of!). In fact, I'd say we spent nearly as much time asking the interviewers questions as they did asking us, so have a few prepared! The student in the interview also asks and answers questions. There's no order in who answers questions, but everyone gets a chance to answer everything.
If you have any other questions other than exactly what the questions they ask are, I'm more than happy to tell you as much as I can. Learning a little of what to expect before the interview started helped me a lot. Not knowing what you're walking into can be intimidating. Just make sure you behave the way you do in the interview throughout the whole day--I'd bet they check a little to see if interview behavior is just an act for anyone.