Users from trusted domain can't access SharePoint farm

I have been given the task to give users from 2 new trusted domains (B & C) access to one of our SharePoint farms.

Currently, these users reside in 2 seperate, but trusted domains.

The "native" users from Domain A are synced by a User Profile Service Application. After adding a new sync connection to the forest of B the crawl runs, but adds 0 new user profiles. I guess the user profile sync account doesn't have the appropriate permissons on B to do the sync.

What I am able to do, is add B\domain users and C\domain users to the visitors group of a Site Collection. However, this doesnt grant them access and results in an Access denied every time.

Besides forwarding this MSDN article to our Exchange admins, which steps can I do, to grant those users access (possibly without user profile sync?)