AFAIK, the shutdown command requires active directory. For workgroup computers, I suggest using PsTools. Grab them from Microsoft, then write a batch file to shutdown a computer. That's what I do, and it works great. You use the psshutdown.exe file to remotely shut down any computer on your workgroup, like this:
There are tutorials on the 'net on how to use psshutdown. You can also open a cmd prompt, navigate to the PsTools directory, and type psshutdown ? for a complete list of commands.