Automatic
groups are different than when people are auto added based on email.
The Automatic groups are
trust_level_0
trust_level_1
trust_level_2
trust_level_3
trust_level_4
staff
moderators
admins
Those you cant add users too directly. Instead you need to give them that specific trust level, moderator or admin. The staff group is admin or moderator. See this post for more info on that.