title: combine multiple rows in one cell based on groups
body: <p>I have a csv file format like this:</p>
  <pre><code>user1 , group1
  user2 , group1
  user3 , group2
  user4 , group2
  </code></pre>
  <p>I want to use python to have the following result:</p>
  <pre><code>group1, user1 user2
  group2, user3 user4
  </code></pre>
  <p>How can I do it?</p>