I interviewed on Wednesday, and I think I remember them saying that there were a large number of seats left. I forget the actual amount.
USC's interview was a weird one for me. There were a lot of people there (almost 30 I think) and the PBL session. I disagree with what someone said earlier that the interview doesn't really matter. I interviewed last year as well, but I didn't get in. I'm almost certain it was because I stayed quiet for the most part during the mock session.
There will be two staff members in your group, and they are taking notes on your behavior.
I thought the tips one of the students there after the session and during lunch gave were worth mentioning, but of course take it with a grain of salt as it is my memory and he's not on the committee.
Do: address people by their names, encourage, speak up, be polite
Don't: be overly aggressive, cut people off, be too passive.
They say they want to see if you'll be right for the PBL curriculum, which is what they do in the session by assessing how well you contribute to the dynamic of the group.