We are members of many groups to which we ascribe the performance of intentional actions, and belonging to these groups seems to give rise, in many cases, to special normative relations. For instance, as a member of the university I have certain duties, as a member of the football team I have others, and yet I have other duties qua member of my law firm. But what is special about some groups that claims of the form "I am under a duty qua member of the group" seem adequate? This paper claims that the standard answer to this question faces two main difficulties. Firstly, most accounts appeal to one special normative notion (e.g. the idea of a joint commitment, or an agreement) to explain such relations, a notion such that, if instantiated, it gives rise to duties that are independent of the value of the joint action. But there are cases where participants think that they are under a duty qua members because the joint activity is valuable, and only because it is valuable. Secondly, most accounts seem unable to explain disagreements among participants about the content of their duties. The paper proposes a model of group action and of normative relations among participants that attempts to overcome both difficulties.