Comment 3 for bug 1072846

Testing with the "User Bar" theme:

When you create an additional user (say "test", password "test") with default settings (groups etc.) in the "User Management" KCM, (only) this user is displayed on the login screen, can be selected and is able to login.

"kubuntu" user ID: 999
"test" user ID: 1000

-> Assumption (note: I do not really know LightDM/AccountsService (config)):
The login screen hides users with UIDs below 1000 (to hide system users), which includes "kubuntu".
The minimum UID of users to show might come from /etc/login.defs via accountsservice (which overrides /etc/lightdm/users.conf's minimum-uid setting, according to its header comment); see accountsservice's src/daemon.c (this is just a guess though!).