/etc/gpasswd – administer the /etc/group file
gpasswd -a user group
gpasswd -d user group
gpasswd -R group
gpasswd -r group
gpasswd [-A user,…] [-M user,…] group
gpasswd is used to administer the /etc/group file (and /etc/gshadow file if compiled with SHADOWGRP defined). Every group can have administrators, members and a password. System administrator can use -A option to define group administrator(s) and -M option to define members and has all rights of group administrators and members.
gpasswd called by a group administrator with group name only prompts for the group password. If password is set the members can still newgrp without a password, non-members must supply the password.