Being responsible is part of a leadership role-just because someone is in a team and not in charge -doesn't make them less a 'leader'- sometimes a good leader must let others excel. Being a good leader and choosing to let others lead is not a bad idea but not participating is a sign that that person needs to learn what a leader must do. I don't excuse it because of being introverted. I don't excuse it for not being the smartest. Assuming the others have good leadership traits, they will help that person move forward, if they really want to. If they choose not to, they lack the skills needed to be a 'team' member.
