The registered users view looks like it is broken. I never used it so I never noticed when I was doing work with permissions.
A party can be a member of a group with more than one relationship. If a user has an admin relationship, it is a subtype of membership_rel and the user will have an entry for both in the group_member_map.
I think using group_id=container_id is the right way to fix this.
When I created a group model all my queries used group_member_map and usually did need the where group_id=container_id clause.
When doing permissions queries you may also need to limit by the rel_type which is what Peter seems to be seeing in the query he tested.
