title: howto change group ownership of directory with normal permission
body: <p>In Ubuntu an user (user_a belonging to group_a) creates a new directory 'new_dir'.
  The ownership of this directory is ofcourse user_a.group_a
  The user is also part of group_b</p>
  
  <p>My question is, why can user_a not change the group ownership to group_b (without using sudo or su) although user_a is part of group_b as well?</p>
  
  <pre><code>chgrp user_a.group_b new_dir results in an error
  </code></pre>