If you want to add domain users or groups to a local group on a Windows client machine automatically, this can be done using group policies. One reason could be to easily put groups or users to the local group Remote Desktop Users to allow them to log on via RDP. To control which users or groups you want to add create a new GPO in the domain and go to Computer configuration > (Policies) > Windows settings > Security settings > Restricted groups.
Once there choose to add a group and in my example find the “Remote Desktop Users” group and after that add the user or group you want to add to the local machines which that particular group policy object applies to. More information about restricted groups can be found at http://support.microsoft.com/?id=810076