@kfogel And you are describing the situation where the team has a visible existence (similar to a user) in the system (for example be listed as the author of a post), I take it? That is, more than just permissions. The more I think about it the more I start thinking of examples: Facebook pages, GitHub teams (especially when a team is tagged as a requested reviewer). I don't remember seeing a name, but "first class teams"? "teams as entities"?
@kfogel Or if you are talking about permissions specifically, Role Based Access Control (RBAC). I think being able to assign multiple roles to a user is part of RBAC, at least it is part of most RBAC systems I have worked with.