Group interviews sound horrible. I just got back from a regular interview weekend and the first night I went to dinner with the other girl trying to get the same spot in the lab, our potential adviser, and a few other people. I felt like I was monopolizing the entire conversation, but the other girl barely spoke the entire time! Turns out she was kind of just like that, she was definitely a better-on-paper kind of person and didn't have much to say. Before I left they told me I was unofficially accepted (and there is only room for one), so I would say it worked to speak my mind whenever I had something intelligent and appropriate to say, but of course I didn't interrupt her or not give her a chance to speak or anything. I can't imagine having to do that with several other candidates as well...good luck!