Normally when you install your ubuntu , you can see the registred users in your system , but exit a way to make it secret .
if you want to disable the userlist in the GDM Login screen, for security reasons, follow these steps
1. Logout of your current session and return to the GDM
2. Switch to the tty command line prompt using Ctrl-Alt-F1
3. Login using your normal login/password
4. at the command line prompt type: export DISPLAY=:0.0
5. then type: sudo -u gdm gconf-editor
6. Switch back to the gdm screen using ALT-F7
7. The Configuration Editor should be loaded.
8. Go to apps/gdm/simple-greeter.
9. Change the Value of disable_user_list to TRUE.
10. Close the Configuration Editor.
11. Reboot your machine, there will be no user list in the GDM.