Check to make sure that your DNS is set up properly.
sudo changeip -checkhostname
Also... Have you bound the clients to the server? You have to do this in the System Preferences under Accounts in 10.6 and higher.
It's in the Directory Utility in 10.5
Once the client computers are bound to the server, you can start creating your User list in Workgroup Manager.
Unfortunately, there isn't a super easy way to automate this. You have to set up your groups, set up your group preferences (if you want any), add your users, and apply them to the groups.
There are some tricks to making this easy, ie set up a user template... That way all you have to do is type the user names and passwords (if you want them).