When an employer has a lot of applicants to interview they may begin the process of weeding out the less desirable ones by conducting group interviews. In a group interview you will be interviewed at the same time as other job candidates. The interviewer or interviewers will address questions to the group as a whole or to individuals within the group.
In all groups there are people who are leaders and people who are followers. Which candidates take on which roles will become evident as the interview progresses. In many instances that is exactly the information the employer needs to help him make his decision. He wants to know who is a leader and who is a follower and will decide who to hire based on this information. All you can do is be yourself. Remember, the job needs to match your personality for you to be successful and happy doing it. With that said, remember to speak up even if your natural tendency isn't to be a leader. You want your voice to be heard, since your goal is to let the employer know more about you.