Evolving set of features that give site/group admins more control over groups
Settings -> BP Group Control
Go to the group homepage, or My Groups, and click "make identifying." You have to be a member of the group to do this.
Go to the group/manage members tab. To add a new user, click "add." To add an existing user, click "add existing." You can get to these screens when you are creating a new group as well.
This functionality isn't really available, however, you can change the language of the message directly in the plugin at line 477 of bpgc-core.php. Remember that if you do this and then upgrade to the next version of bp-group-control (which will let you create your own template), it will erase your changes.