I am currently setting up a new website that will probably have between 3-20 sub-domains. Each sub domain will need a forum.

In each sub-domain, I need to either set up a forum, or preferably, set it up so that I simply install 1 instance of miniBB and just show specific forums in each sub-domain.

Ideally, users would register at the main site, then simply log into the forums on each sub-domain. So far, I have not found a solution other than installing an instance on each sub-domain.

No, I do not want to have all the forums visible except to an admin.
Yes, I want the help forums visible from the main site.

From any sub-domain I want to define what forums will be visible.

Is this possible to acheive with a single install of miniBB or would I be better off using multiple installs.

In past time, I've did something like that for a custom forum. The have had one common installation under www.somedomain.com, and using quite simple dynamic solution, it was possible to load subdomain1.somedomain.com, subdomain2.somedomain.com with the same installation and users table. So users registering at one forum, automatically are registered at another (all) forums. The only thing is not possible is to show all forums at once for admin. At the same time, it's possible to prepare a list of all domains and check them all, visiting from this list.

This solution is paid, if you don't know how to implement it yourself.