The gpasswd command
administer /etc/group and /etc/gshadow 2 examples from the lessons of the IVski Linux Playground — try them in the terminal next to them.
Options in the examples
-a, --add user- Add the user to the named group.
-d, --delete user- Remove the user from the named group.
Examples
sudo gpasswd -a user groupAdd user to group
sudo gpasswd -d user groupRemove user from group