Allowing ProSite members from main site access to another site on network

I have a quick question regards giving users of one site access to another on the network.

In short users will signup for a site on the primary site and then they get their own website thanks to ProSites. We have our support on a subdomain on the same network, we have a forum and want users to be able to login and use the forum using the login details from their own site on the network.

Is there a normal way of achieving this? I don't want to manually add each user, I need it to automatically add the user to the support site.