gpasswd – administer the /etc/group file  setting password 

gpasswd group
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.

Group administrator can add and delete users using -a and -d options respectively. Administrators can use -r option to remove group password. When no password is set only group members can use newgrp to join the group. Option -R disables access to the group through newgrp command.

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.