Add a user account to the local Administrators group :

The following powershell commands add the given user account to local Admin group.

$user = "ComputerName/user1";
$group = "Administrators";
$groupObj =[ADSI]"WinNT://./$group,group" 
$userObj = [ADSI]"WinNT://$user,user"
$groupObj.Add($userObj.Path)

Add a AD domain user account to the local Admin group :

We can use the above same commands to add domain user account by just passing the domain user.

$domainUser = "DomainName/user1";
$group = "Administrators";
$groupObj =[ADSI]"WinNT://./$group,group" 
$userObj = [ADSI]"WinNT://$domainUser,user"
$groupObj.Add($userObj.Path)

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

We need to just pass the remote machine name to add an Active Directory user to the local Administrators group on a remote Windows computer with PowerShell.

$computer = "GEN8";
$domainUser = "DomainName/user1";
$group = "Administrators";
$groupObj =[ADSI]"WinNT://$computer/$group,group" 
$userObj = [ADSI]"WinNT://$domainUser,user"
$groupObj.Add($userObj.Path)