Add a user to the local Administrators group

In local computer

$user = "ComputerName/Morgan";
$group = "Administrators";
$groupObj =[ADSI]"WinNT://./$group,group" 
$userObj = [ADSI]"WinNT://$user,user"
$groupObj.Add($userObj.Path)

Add a domain user account to the local Administrators group on a Remote computer

$computer = "hp-pc";
$domainUser = "DomainName/Morgan";
$group = "Administrators";
$groupObj =[ADSI]"WinNT://$computer/$group,group" 
$userObj = [ADSI]"WinNT://$domainUser,user"
$groupObj.Add($userObj.Path)