Creating trust relationship in Windows

SharePoint is installed in Domain1 and you want to allow Domain2 users to access SharePoint. Both domains are in different forests. The solution is to create a 1 way trust between the two domains. Domain2 should trust Domain1 which is in login forest. To learn how to create trust relationship, see the following article:

http://www.windowsnetworking.com/articles_tutorials/Creating-Trusts-Between-Forests.html