The chmod command is used to change the permissions of files and directories in Linux. It can grant or revoke read, write, and execute permissions for the owner, the group, and others. To change the file permissions to only allow access to a certain group of users, the chmod command can use either the symbolic or the numeric mode. For example, to give read and write permissions to the group and no permissions to others, the command can be:
chmod g+rw,o-rwx filename
or
chmod 660 filename