Don't list all users at login with LightDM

You can do this by editing /etc/lightdm/lightdm.conf The default contents are just:

[SeatDefaults]
greeter-session=unity-greeter
user-session=ubuntu

And you can just add:

greeter-hide-users=true

To that. For the full list of options, see /usr/share/doc/lightdm/lightdm.conf (or look at this post about lightdm conf).

Answers suggesting you edit the conf file are wrong.

From the Ubuntu wiki:

LighDM configuration is governed by the /etc/lightdm/lightdm.conf file, however it's not suppose to be directly edited, instead use:

lightdm-set-defaults

I found the file in /usr/lib/lightdm/lightdm-set-defaults.

You would issue the following on the command line:

sudo /usr/lib/lightdm/lightdm-set-defaults --hide-users true