gpasswd - administer /etc/group and /etc/gshadow
[SYNOPSIS] gpasswd OPTION GROUP
option
-A
Set the list of administrative users
# gpasswd -A user group
-a
Add the user to the named group
# gpasswd -a user group
-d
Remove the user from the named group
# gpasswd -d user group
-r
Remove the password from the named group
# gpasswd -r group