I had a client that needed to add a couple of users to the local administrators group of every machine on the domain.  After looking through the options I found this solution as the easiest and least likely to create other permission issues on the computer.

Simply adding users to the existing

1. Create a security group called “Deskside Support” and add the appropriate users to it. 
2. Configure “Deskside Support” as a Restricted Group.  
3. On “This group is a member of the following groups…” add “Administrators.”  
4. This will add “Deskside Support” to the Administrators group on any computer that this GPO applies to, without removing any other members that are already present.