Proper way to manage a collection of existing objects

Hello,

I've been looking for a pattern to implement managing a collection of exiting objects, and haven't had much luck.

For example, if I have REST controllers for users, and for groups, what would be a good way to implements the assignment of users to groups, where the group was already selected? Would it be to add the functionality to the existing group controller? Create a new controller to manage the association of the two?

I've seen plenty of examples of writing RESTfull controllers for individual models, and examples handling the creation of new dependent models (blog posts with new comments), but I haven't been able to find examples of more complicated interactions. Does anyone know of a good reference?

Thanks.

-Michael