If you go into settings: There is not a “staff only” setting that I’m aware of, but if you also check ‘must approve users’ this will ensure that if they aren’t invited by a staff they can’t join unless they are approved by staff./admin/site_settings/category/all_results?filter=invite you can set the 'default invitee trust level` and crank it up to 4.
if you go to /admin/users/list/leader you can see everyone who has trust level 4.