How to change an XP to join a domain

I have a consultant who needs access to shared resources on our network. I have added him to the user list, and configured his PC for a static IP address, however, when he logs on to his PC, he cannot specify our network domain name. He can see the computers on the network but each shared resource asks for a userid and password. Specifying valid passwords still does not give him permissions to the resources. How can I get his XP to join the domain?