List, set and change standard ugo/rwx permissions

Modify user
# usermod -g groupname user_to_be_added

Change group ownership
#chgrp root filename

Change ownership
# chown username.groupname filename

Change permission
#chmod xxx filename
(where xxx can be digits like 644, 755, etc representing user,group and other permission)

or
chmod u=rwx,g=rx,o=rx filename

List permission
#ls -l