I suggest you check out www.interviewfeedback.com Also, I have additional interview resources available on my Links page. I don't know what that university's group interviews are like, so www.interviewfeedback.com is your best bet.
However, there's two possibilities that I know of. An interview where it's you being interviewed by more than one interviewer. For example, U of Sask has four interviewers in a room with you, and U of Calgary has two interviewers interviewing you simultaneously.
An interview where there's more than one interviewee in the room. For example, at McMaster, a small group of interviewees are placed in a room and asked to solve or discuss a problem/issue, while being observed and evaluated by interviewers.
I believe this is to gauge your comfort level working and interacting with a group of students, which in my opinion is absolutely crucial to success in a Problem-Based Learning (PBL) curriculum. As with all interviews, be yourself, and stay relaxed. PBL really demands respect for your peers, and a genuine interest in helping the group along, even on those bad days in medical school when you're sick, haven't been sleeping, and didn't have time to stop for coffee before class. Make sure to remember that during your interviews, and when deciding which school is right for you.