If you want a user to NOT show up at the OS X login screen (i.e. not have a button and name appear for them), do this:
1. Create the user if you haven't yet ("Users" in the system preferences)
2. Open NetInfo Manager (in /Applications/Utilities)
3. Authenticate by clicking the lock icon and typing in your admin password
4. In the middle column of the column view at the top of the window, scroll down to and select "users".
5. Select the user in question from the list that appears to the right
6. In the bottom of the window click the "property" called "realname", such that the entire line is hilighted, not just the text of the actual name.
7. Press delete or click the delete button and confirm the deletion.
8. Click the users name in the column view at the top and when prompted save your changes.
9. Quit NetInfo Manager
Now if you logout, that user will no longer be listed but you can still login as them via an SSH session or via the "Other User" button if you have that button enabled (in the "Login" system preferences). Of course, you must use the short name when logging in as there is no longer a "real name" for this user.
