Community forum only for site admins and users


I want to create as a subsite hosting the forums for the whole multisite so the forum would be at

But I want the forums ONLY to be accessible/editable for already-registered site admins, registered elsewhere on the site eg site owners of

and for site users of those sites.

ie the forum is not accessible by normal registration at

Instead they must already be registered as a site admin or site user elsewhere on the

What is the best way to enable this?