Several ways, you can give them the "add workstations to domain" right using a group policy
http://technet.microsoft.com/en-us/library/cc780195(WS.10).aspx You can also use the delegation control wizard
John covers both methods here:
http://www.windowsitpro.com/article/domains2/jsi-tip-8144-how-can-i-allow-an-ordinary-user-to-add-a-computer-to-a-domain-.aspxI've seen both methods used...where I am right now we use method 1
Thanks
Mike