I was doing some test with USERGROUPS_ENAB under /etc/login.defs

If I set the value to no Then useradd will not create a default group with the name of the user (Great !)

instead it will assign the group users that exist by default.

If I substitute another user that not in the users group and do

su another user
newgrp users
Password:

I get prompted for a password, I've tried everything, root psw, the suer password, blank, but nothing work..
Is that mean that user can't add themselves to a group ?

Thanks