By default the user list isn't showen on the login screen. But it can be added by edeting the login window theme.

The themes are located at /usr/share/mdm/themes/

I use the linuxmint theme, so I edited the theme.xml in the linuxmint folder. You've to open it as root. So it's something like this (I use gedit as editor, it's not installed by default!):

sudo gedit /usr/share/mdm/themes/linuxmint/theme.xml

To add the user list you have to insert

<item type="list" id="userlist">
     <pos x="150" y="200" height="110" width="250"/>
     <color iconcolor="#ffffff" labelcolor="#ffffff"/>
</item>

in the file. If you use another theme, you might have to change the values and the position in the file.

I placed it right above the username and password text box (<!-- username/password label & entry container -->)