Adding domain user to local admin group

I have a windows 2000 server and all workstations running 2000 pro. I am trying to add a domain user account to the local admin group on the workstation so that the client has full control of their own pc. The workstation is already a member of the domain as well as the user account. I know that the workstation and user are both able to be authenticated to the domain. The problem I am having is that when I try to add the domain user to the local admin group I recieve this error…..”unable to process object the object either does not exist or the domain could not be contacted.” I have never had this problem before. I know that I am authenticating to the domain because I was able to login as the domain user and have the workstation added to the domain. Does anyone know the solution? Help is greatly appreciated!