Groups

Group membership. A group IS a principal, so it is created and listed under Principals.

Operation	Auth	Purpose
GET /api/groups/{gid}/members	Bearer + DPoP	List a group's members.
PUT /api/groups/{gid}/members/{pid}	Bearer + DPoP	Add a principal to a group. Idempotent.
DELETE /api/groups/{gid}/members/{pid}	Bearer + DPoP	Remove a principal from a group.