Congrats on getting the interview!
There is a group activity, but I'm sure you'll do great.They make it as non-stressful as possible. You'll work with your group to come up with a solution, and make a presentation to the other group. Remember, you've got the support of your team and the audience.
You may be able to finish around 4:30pm at the earliest, and I would recommend letting one of the administrators at UB know your situation, so they can better accommodate your time. (FYI: After 4:30pm, there's a social/refreshments and an optional tour.) UB is really friendly and understanding. I remember they asked if anyone needed to catch a flight that day, so they'll understand your situation. When is your interview?
Best of luck to you!