In liberal political philosophy, a prevalent view holds that groups are typically voluntary associations. Members of voluntary associations can accept, revise or reject group practices as a matter of choice. In this article, I challenge this view. Appealing to the concept of joint commitment developed in philosophy of social science, I argue that individuals who jointly commit their wills to a goal or a belief form a ‘We’-group. Members of ‘We’-groups are under an obligation to defer to ‘Our’ will embodied (...) in ‘Our’ norms as a matter of course. I further show the ubiquity of We-groups. This joint commitment account of group authority raises a much-overlooked question of group legitimacy: Do members have good reasons to obey norms of their group? I show that state-centric views of legitimacy are inapt to answer it. A group-centric view, revived from the old communitarian literature, is defended. (shrink)