You meet in a place, where there are some snacks/drinks alcoholic/non-alcoholic. There are residents, some faculty, other interviewees
people wear nice clothes, not too casual, not very professional.
You just talk to others and the residents, and ask questions about the program, be pleasant, friendly, don't be obnoxious, don't exclude others.
Try to be social even if you are a shy person. I bet the residents will have a say who they like too.
so it's unofficial part of the interview, so don't say silly stuff you'll regret later.
I found arriving 5 mins earlier make you get to know others better, arriving on time, and later, makes you more exlcuded from conversations because everyone by then have already introduced themselves and got to know each others better.