There's an app called SharePoints that has the ability to create share-only users (no home folder).
I believe you could also do this in NetInfo Manager by creating a user and not specifying a Home Directory. You'd probably also want to give the user a UID below 500 so they wouldn't show up in the Login Panel if it was set to show a list of users on the system.
I personally would go with SharePoints. I hate mucking around in NetInfo.