title: Groupby and string-concatenate multiple columns
body: <p>I have a dataframe like so:</p>
  <pre><code>ID  CODE  GROUP  PIN
   1   99     A     221
   1   89     B     443
   2   79     A     230
   2   69     A     000
   3   59     D     781
   3   49     T     665
  </code></pre>
  <p>I want to groupby the ID column and string-concatenate all the other fields, comma-separated, so I get output like so:</p>
  <pre><code> ID  CODE     GROUP      PIN
   1   99,89     A,B     221,443
   2   79,69     A,A     230,000
   3   59,49     D,T     781,665
  </code></pre>
  <p>I can <code>groupby</code> the ID and return the CODE field concatenated but I need to do that for more than just one field:</p>
  <pre><code>df = DATA.groupby('ID')['CODE'].apply(',' .join).reset_index(drop = False)
  </code></pre>
  <p>This returns just the grouped ID field and a concatenated CODE field. How can I expand the groupby to multiple columns?</p>
  <p>Many thanks</p>