[CmdletBinding()]
Param(
     [Parameter(Mandatory=$True,Position=0)]
     [alias("user","name")]
     [string]
     $UserName,
     [Parameter(Mandatory=$False,Position=1)]
     [string]
     $ComputerName = 'localhost',
     [Parameter(Mandatory=$False)]
     [string]
     $Domain = 'WORKGROUP'
     )

$Group = 'Administrators'

# Please be warned. The syntax of [ADSI] is CASE SENSITIVE!
$target=[ADSI]"WinNT://$ComputerName/$Group,group"
$target.psbase.Invoke("Add",([ADSI]"WinNT://$Domain/$UserName").path)