title: Making a group in dataframe in pandas
body: <p>I have a list such as<br/></p>
  
  <pre><code>groups = [['Group1', 'A', 'B'], ['Group2', 'C', 'D']]
  </code></pre>
  
  <p>and a dataframe such as</p>
  
  <pre><code>A 100
  B 200
  C 300
  D 400
  </code></pre>
  
  <p>I want to make a group sum from the list above to become:</p>
  
  <pre><code>Group 1 300
  Group 2 700
  </code></pre>
  
  <p>How can I do this using python pandas?
  Needless to say I am a newbie in pandas. Thanks.</p>