Fixed Group.member_users and member_groups to work with empty groups

The Group.member_users and Group.member_groups methods would fail when
empty groups are present in the hierarchy.  Changed the access method
to [:member] to return an empty array instead.
Merge branch 'fix_group_member'
Return only unique users/groups with member_users/groups

Add uniq to member_users and member_groups methods so users/groups are
returned only once.