Adding Users from Other Domains in the Forest

Hey all,

I'm new to AD and I'm learning the ropes.  I have two domains in a forest.  How the heck do I add users in one domain that already exist in the other?  What I'm looking to do is add an admin from one domain to the Domain Admins Group in the other.  Is this even possible?  When I go to add, the dialog box limits it to Contacts and Other objects.

Thanks for any assistance!