Joining computers to the domain

I maintain a couple small business servers in different companies. I infrequently add a new workstation to a domain, so whenever I do it, I need to look up information on how to do it. The one thing that always confuses me is how to add an existing domain user to the workstation. I assign myself (a domain admin), the primary user of the computer, and sometimes another user to the workstation.  I know how to find them in the list and assign them to the computer, but usually some user gets 2 accou