Create a hidden user
Try giving the user a UID below 1000. This makes Ubuntu treat the user as a system user, which then won't show up on the login screen.
Add the user in whatever way you prefer, and then run:
Code:
sudo usermod -u 6996 username
sudo usermod -a -G admin username
Where 599 is an unused UID below 1000, and newuser is your new user. The first command changes the UID and the second adds the user to the admin group, granting them sudo rights.
