CRUD Conventions for Many to Many Relations

I am curious is there is a convention for the the following in CRUD:

I have users and groups. Each group can have many users, and each user
can have many groups.
These are related through a has_many :through pattern, with the
connecting model being membership.

The most common uses of these data are to get a list of groups for one
user, or a list of users for one group, or to check if two users are
in the same group.

What would be the best way to set up the controllers so that these
uses are easy and intuitive. Or, what setup would make it easiest for
the next developer to grok the system when I get hit by a bus.

Regards,

Eli

Wow,

You just opened up a whole new world for me. Thank you.