Community forum only for site admins and users

Hi,

I want to create
domain.com/community/ as a subsite hosting the forums for the whole multisite so the forum would be at
domain.com/community/forum/
But I want the forums ONLY to be accessible/editable for already-registered site admins, registered elsewhere on the site eg site owners of
domain.com/site1
domain.com/site2
and for site users of those sites.

ie the forum is not accessible by normal registration at
domain.com/community/forum
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?

Thanks