Since my original answer got transformed in a comment due to it's brevity, here is another go.
The command that you are looking for is
dscl on OSX (which works both in OS 10.6 and 10.7 in my experience). As I mentioned this question has been asked before, and you can find the original answer (plus usage examples) on stackExcjange superUser forum at:
which provides a number of examples for its use. For instance, adding a user to a group would work as:
dscl . append /Groups/the_group_you_want GroupMembership the_user
In addition, someone did provide a bash script that does what usermod does on linux, and you can find it here:
(I haven't tested it, so it comes with no guarantees).