I just had a group interview on Friday and one thing I really noticed was people stepping on each others toes to try to be impressive. My advice: take space, make space. If they ask you to go around and share something, you don't need to give your entire life story and philosophy no matter how interesting it may be. If you're in a session about diversity, don't play into the oppression olympics, and if you know you have a lot of privilege it is much better to acknowledge that than to try to dig up some experience of marginalization. In terms of questions: you'll likely be with faculty that you'll never work with in the group interviews. Ask big picture questions about the program as a whole (how do students fare on the job market, what are the strength and weaknesses of the program, what is the general life contentment of students), and save the nitty gritty detail questions (how does summer funding work, do grad students collaborate on projects, where is most data collected) for interviews with your PI. I hope this helps! All in all, just be present and try to show some personality and awareness.