gpasswd

Administer "/etc/group" and "/etc/gshadow".

Define group administrators:

sudo gpasswd -A {{user1,user2}} {{group}}

Set the list of group members:

sudo gpasswd -M {{user1,user2}} {{group}}

Create a password for the named group:

gpasswd {{group}}

Add a user to the named group:

gpasswd -a {{user}} {{group}}

Remove a user from the named group:

gpasswd -d {{user}} {{group}}