Ubuntu: User ids under 1000 are hidden

Any UID values below 1000 are treated as system type users. These are hidden from the Login-screen.

If you want to see users on the Login-Screen the UID value needs to be greater than 1000.

Otherwise edit /etc/login.defs and search for UID_MIN in the text file. Change that value from 1000 to something smaller like 501

# vi /etc/login.defs

UID_MIN                  500
GID_MIN                  500