NAME： gpasswd - administer the /etc/group file
SYNOPSIS：
gpasswd group
gpasswd -a user group
gpasswd -d user group
gpasswd -R group
gpasswd -r group
gpasswd [-A user,...] [-M user,...] group
OPTIONS：
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 via a password to the group through newgrp command (however members will still be able to switch to this group).