title: Can't call column names in a dataframe after GROUPBY
body: <p>Sorry about the formatting.  Can't seem to figure that out.</p>
  
  <p>Unable to refer to column names in a grouped dataframe.     </p>
  
  <pre><code>dfbidFeed       
  
  MARKET  30D_PCLICK      
  Dallas       5      
  Dallas       4      
  Houston     10      
  Houston     13      
  New York     7      
  New York    12      
  St. Louis   15      
  St. Louis   14      
  </code></pre>
  
  <p>Then I run this code to compute the mean and the sum of <code>CLICKS</code>, grouped by <code>MARKET</code>   </p>
  
  <pre><code>dfclicks_grouped = dfbidFeed['30D_PCLICK']
                     .groupby([dfbidFeed['MARKET']])
                     .agg({"returns": [np.mean, np.sum]})     
  </code></pre>
  
  <p>returns:</p>
  
  <pre><code>MARKET       mean   sum     
  Dallas       4.5     9  
  Houston     11.5    23  
  New York     9.5    19  
  St. Louis   14.5    29         
  </code></pre>
  
  <p>I want to be able to merge <code>dfclicks_grouped</code> with another <code>df</code>.  However, I can't seem to refer to the columns in <code>dfclicks_grouped</code>.<br>
  It doesn't behave like a dataframe where I can refer to the column name and get a result.  (i.e. <code>dfbidFeed['MARKET']</code> returns the list of markets.     </p>
  
  <p>When I do this (<code>dfclicks_grouped['MARKET']</code>, I get:  <code>keyError:  'MARKET'</code>     </p>
  
  <p>How do I make the colunn names in <code>dfclicks_grouped</code> callable?      </p>