Here is a simple Powershell script that I created, if you need to add domain computer accounts, users or groups to the local administrator group.
The script should be run locally on the computer on which you want the accounts to be added to the local administrator group.

 
 #Add Computers/Users/Groups to the local administrator group
 # Version 1.0
 #Date created: 2013-08-29
 #Created by: Daniel Classon

 $domain="<domain>"
 $computername= "$env:computername"
 $group=$computer.psbase.children.find("administrators")

function AddToGroup($number)
{
     $group.add("WinNT://"+$domain+"/"+$number )
}

#Add these domain users/groups to the local administrator group
 AddToGroup "<group>"
 AddToGroup "<group>"
 
#Add these domain computer accounts to the local administrator group. Computer accounts always end with $.
 AddToGroup "<computer account>$"

REFERENCES

RELATED POSTS