I want to create
domain.com/community/ 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 domain.com
What is the best way to enable this?