Received from Mark Fletcher at 2/11/2019 04:00 PM UTC:
What do other people think about this? Should this be a new/different permission? The permission is currently just worded 'Invite Members'. Previously a moderator of a subgroup who had this (but not in the parent group), could only add existing members of the parent group to the subgroup (directly adding them). Now they can invite people who are not members of the parent group to join the subgroup (they would be added to parent group automatically on joining).
Since members of a subgroup must be a member of the main group, a subgroup moderator with no permission to add to the main group should not be able to add a member to the subgroup.
I suppose you could add a new permission the Group Owner could apply to subgroup moderators that would permit the subgroup moderator (who isn't a moderator in the main group) to add members to the main group ONLY by adding them to the subgroup, but that's starting to feel a bit messy for my taste.