To grant a user local administrative privledges on their workstation, you would add them to the local Administrators group on their workstation. They will still be ordinary users in the domain, but will simultaneously be an Administrator on their Windows NT Workstation.
You can do this without visiting their workstation, if the Schedule service is running on the workstation, either under the System Account (the default) or under an Administrators or Domain Admins Account. On your workstation (or server), open a CMD prompt and type:
AT \\RemoteComputerName HH:MM cmd /c "net localgroup Administrators DomainName\UserName /ADD"
where HH:MM is at least 2 minutes from now, or if you have the resource kit
Soon \\RemoteComputerName 61 cmd /c "net localgroup Administrators DomainName\UserName /ADD"
