title: pandas - keep only True values after groupby a DataFrame
body: <p>I've been working on a DataFrame with User_IDs, DateTime objects and other information, like the following extract:</p>
  
  <pre><code>User_ID;Latitude;Longitude;Datetime
  222583401;41.4020375;2.1478710;2014-07-06 20:49:20
  287280509;41.3671346;2.0793115;2013-01-30 09:25:47
  329757763;41.5453577;2.1175164;2012-09-25 08:40:59
  189757330;41.5844998;2.5621569;2013-10-01 11:55:20
  624921653;41.5931846;2.3030671;2013-07-09 20:12:20
  414673119;41.5550136;2.0965829;2014-02-24 20:15:30
  414673119;41.5550136;2.0975829;2014-02-24 20:16:30
  414673119;41.5550136;2.0985829;2014-02-24 20:17:30
  </code></pre>
  
  <p>I've grouped Users with:</p>
  
  <pre><code>g = df.groupby(['User_ID','Datetime'])
  </code></pre>
  
  <p>and then check for no-single DataTime objects:</p>
  
  <pre><code>df = df.groupby('User_ID')['Datetime'].apply(lambda g: len(g)&gt;1)
  </code></pre>
  
  <p>I've obtained the following boolean DataFrame:</p>
  
  <pre><code>User_ID
  189757330    False
  222583401    False
  287280509    False
  329757763    False
  414673119     True
  624921653    False
  Name: Datetime, dtype: bool
  </code></pre>
  
  <p>which is fine for my purposes to keep only User_ID with a True masked value. Now I would like to keep only the User_ID values associated to the True values, and write them to a new DataFrame with <code>pandas.to_csv</code>, for instance. The expected DataFrame would contain only the User_ID with more than one DateTime object:</p>
  
  <pre><code>User_ID;Latitude;Longitude;Datetime
  414673119;41.5550136;2.0965829;2014-02-24 20:15:30
  414673119;41.5550136;2.0975829;2014-02-24 20:16:30
  414673119;41.5550136;2.0985829;2014-02-24 20:17:30
  </code></pre>
  
  <p>How may I have access to the boolean values for each User_ID? Thanks for your kind help.</p>