Linux provides two programs to help you manage user and group ownership from the command line. The first, gpasswd, allows you to administer a group with relative ease. chgrp, the second program, allows users to change the group of particular files.
To add users to an existing group, log in as the root user and enter the following command:
[root@insocoxygen /root]# gpasswd -a loginname groupname
where loginname is the login name of the user you want to add to the group groupname.
For example, to add the user vector to the group ittf, you would use the command
[root@insocoxygen /root]# gpasswd -a vector ittf
To remove a user from an existing group, log in as the root user and ...