Below is USERADD man page excerpt :-
-n A group having the same name as the user being added to the system will be created by default. This option will turn off this Red Hat Linux specific behavior. When this option is used, users by default will be placed in hatever group is specified in /etc/default/useradd. If no default group is defined, group 1 will be used.
I need info for the BOLD part, and please post after performing practically.