How to set workstation user permission

You will need to go into the Control Panel at the workstations.
From there you will go into User Accounts and select 'Add...'
This will ask you for a user name and domain to which you will need to enter Domain Users and and click Next.
On the next screen select Other and find Administrators in the select box.
Then click Finish
This should give you what you need.

You don't have to go to each machine. You could assign a startup script to the pc's that will do this for you at next startup. Create a .bat file and put this in it:

net localgroup Administrators /add "domain\Domain Users"

If all the pc's are in the default computers OU you will need to create another, because you can't apply a GPO to this OU. Create a temporary OU and apply this batch file in Group Policy. Move the pc's into this OU temporarly until they get the addition. Then move them back if you wish.