This is a request to extend the work done in Groups should have a root directory page, just like Users and Group owners for managing group membership
Users should be able to make join requests to private groups. The group admins should get notifications for such requests and be able to add the member to the group. This could also be optimised in some ways:
Require minimum number of approvals for addition to the group by the group admins. Useful for groups with many admins.
Allow non-admins to add members to group by voting (with min-voting requirement). This will result in low headache for group-admins and lower wait time for the requester. This also needs that every group member be notified of this and a separate page for approvals.
Would prefer to have admin view-able logs for who granted access to whom.