Think the UDF sites have stuff for this.

Code:
Function AddToLocalAdmin($Computer,$GroupToAddTo,$Domain,optional $GroupToAdd,optional $AccountToAdd)
  Dim $Group,$User,$Admin,$Err
  $Group   = GetObject('WinNT://' + $Computer +  '/' + $GroupToAddTo + ',group')
  If $GroupToAdd<>""
    $User  = GetObject('WinNT://' + $Domain + '/' + $GroupToAdd + ',group')
  EndIf
  If $AccountToAdd<>""
    $User  = GetObject('WinNT://' + $Domain + '/' + $AccountToAdd + ',user')
  EndIf
  $Group.Add($User.ADsPath)
  $Err = @ERROR
  $AddToLocalAdmin=$Err
EndFunction