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3.5.2. Attaching Users to Groups

If you want to add an existing user to the named group, you can make use of the gpasswd command.
gpasswd -a username which_group_to_edit
To remove a user from the named group, run:
gpasswd -d username which_group_to_edit
To set the list of group members, write the user names after the --members option dividing them with commas and no spaces:
gpasswd --members username_1,username_2 which_group_to_edit