It's by no means mandatory. Its purpose is just to make you feel more comfortable with the format. It's simply a casual hangout where you can talk about anything from good food places in lubbock to research opportunities in med school. This school really cares about providing a laid-back, stress-free interview experience, and the virtual hangout is just to help do that. You'll meet the nicest people ever, and the whole thing will not even feel like a med school interview. Good luck. Just be yourself and genuinely enjoy it.