Solidary groups such as social movement organizations are not corporate agencies with clear command structures. Yet they are able to value, deliberate, choose, and act in a way that governs the actions of their individual members, even when the members disagree. How then should we understand the structure of individual and collective agency in solidarity? I shall canvass two well-known schools of thought in the theory of collective action -- the collective-intention approach epitomized by Michael Bratman and the group-agent approach epitomized by Margaret Gilbert -- and show that neither approach as it stands can account adequately for solidarity. Then, emphasizing some distinctive features of action in solidarity, I propose that the most natural analysis of such action follows a practical-reasoning model of team agency along the lines proposed by Natalie Gold and Robert Sugden. Teams are constructed through deliberative processes of organizing, such that acting in solidarity manifests one's recognition of particular others. This result opens up a new account of when and why solidarity is morally obligatory.