Group and subgroup membership

To make it short "a member of a group is a member of all its subgroups, not the other way round".

Be careful when dealing with subgroup membership: subgroups can be seen a seen as "owned" by the parent group, not like it would be a "specialized" group within the group. Therefore, users of a given group are considered also members of all its subgroups (they "own" the subgroups), whereas users of the subgroups are restricted to the subgroups, they are not seen as member of the parent group.