Hello kachbo1,
You can use net localgroup command like this:
[tt] net localgroup administrators sales\ralphr /add[/tt]
mimic-ing an example given in net localgroup documentation adding domain (sales) user ralphr to the (local) administrators group.
Run it on the computer local, not from a remote computer. To do this, run the line from computer startup script for instance. Put those computers which need to do this into a (specially madeup) group is a way and an ingredient to use gpo-driven deployment plan.
regards - tsuji