Assuming the workstations have been added to the domain...
I will show you how to do it without ever touching the client computer.
1. From the domain server oe any machine that has the admin tools installed and domain admin rights open Active Directory Users and computers.
2. Open up the Computers folder in your active directory structure. You will see a list of all your computers that haqve been added to the domain.
3. Right click on the computer that you want to modify security for and select manage.
3. In the Computer Management Window expand Local Users and Groups.
5. Click on Groups.
6. Open up administrators. This is the local administrators group on the machine you are connected to.
7. Click on Add.
8. Make sure that location is set to your domain name or All Locations.
9. Find Domain Users in the list and add it to the local Administrators Group.
It's that easy.
If you do not see the computer in question listed in the computers section of Active Directory Users and computers the machine has not been added to the domain. To add the computer to the domain go log into it locally as the local admin. Right click on My Computer and select Properties. Go to the computer name tab. Click on the change button, select domain and enter the domain name. You will be promted to provide Domain credentials to add the computer. Use the domain admin ID and password.