NET localgroup administrators mydomain\mylocaladmin add
This command will add the user mydomain\mylocaladmin to the local
administrator group on the box from which the command is run. You would have
to run this command on each box somehow -- using the AT command, logging
into each box, script (vbs, js), or some other means.
That being said, you may find restricted groups (mentioned in another reply)
better suit your needs.
\\ MadDHatteR