"JasonB" has written two shell scripts which enable the addition and deletion of users to your OS X system from the command line. Normally this would be done in Netinfo, but the command line option is nice if you ever want to do this remotely.
His scripts and instructions can be found on this page of his web site.
I have not tried these yet personally, but there's no reason to believe they won't work.
Mac OS X Hints