A central focus of group case interviews is to evaluate interpersonal skills. You'll want to put your best foot forward and come across as someone the interviewers would like to work with.
First of all,speak with a purpose, your goal is quality over quantity.
The next tip: Involve everyone during the interview process, help others if you see they're struggling to express their opinion. That's a sign of a leader.
Important! Summarise key points. This can be done during team discussion.
Hope it helps.