I should have reread the original post.
If your group is a component of the main subsite applicaiton group (i think all groups created through the acs-subsite admin pages are) then your users will be a member of the group and any other group the original group is a component of.
So if you want to see members that are direct members of a group and not due to a composite relationship you need to specify group_id=container_id.
I guess it depends on what the registered_users view should be used for if it should be changed or if the current behavior is correct.
