Originally Posted by divyashree
Only root is allowed to change the group ownerships. You must be root to run this command successfully.
Incorrect. The OWNER of a file may change the group of any file, providing they're a member of the group they're trying to change.
If a user belongs to "users, staff, video, cdrom", they can then change group ownership of a file/directory to be any one of those. Any group they don't belong to, needs root access.