You can refer to the relevant discussion in
Bug:488158 users-admin should leave adduser handle main group creation and Bug:488913 users-admin defaults to override system wide defaults from /etc/login.defs, /adduser.conf
You can refer to the relevant discussion in
Bug:488158 users-admin should leave adduser handle main group creation
and
Bug:488913 users-admin defaults to override system wide defaults from /etc/login.defs, /adduser.conf