Hello,
Unfortunately, there is no possibility to grant the permissions the way you need. It is only possible to grant the permissions to modify the Member property of the group, not limit the members that can be added. The approach with a Business Rule triggering Before adding a member to a group is exactly what you need. In the rule, you will need to use two conditions to check whether a user is adding themselves to the group and whether the initiator is the owner of the group. Finally, the Business Rule will look like the following: