Go to the user's workstation. Log on as an admin of that workstation or the domain admin. Right click on my computer, click on manage. Expand local users and groups, click on groups, double click on administrators, click add, then click advanced, then find now. At this point it should be displaying all domain accounts, find and select the user you want to add. Click ok three times.
Network Admin
A+, Network+, MCSA 2000, MCSE 2000