BuddyPress.org

I’ve to manage a community of young children. I disabled the ability for them (“subscribers”) to create a group in BuddyPress Settings.
When a child wants to belong to a group, he/she posts a mention to “ @admin” asking to become a member of Group X, right.

Problem is: How can I, as the administrator from within the Dashboard, make the child a member of the requested group?
Thanks.