Group passwords are an inherent security problem since more than one person is permitted to know the password. However, groups are a useful tool for permitting co-operation between different users.
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).
gpasswd called by a group administrator with group name only prompts for the group password. If password is set the members can still newgrp(1) without a password, non-members must supply the password.
newgrp(1), gshadow(5), groupadd(8), groupdel(8), groupmod(8), grpck(8), group(5).