Changing file ownership

You can change group who owns a file if
You are root, or

You own the file, and
You want to change the group to a group of which you are a member

 $ id
 uid=1001(reegen) gid=500(seattle) groups=500(seattle),5002(web)
 $ ls -l somefile
 -rw-rw-rw-  1 reegen seattle      0 Apr 19  2000 somefile

 $ chgrp web somefile; ls -l somefile
 -rw-rw-rw-  1 reegen web          0 Apr 19  2000 somefile

 $ chgrp cgi somefile; ls -l somefile
 chgrp: Operation not permitted
 -rw-rw-rw-  1 reegen web          0 Apr 19  2000 somefile