title: Replicating GROUP_CONCAT for pandas.DataFrame
body: <p>I have a pandas DataFrame df:</p>
  
  <pre><code>+------+---------+  
  | team | user    |  
  +------+---------+  
  | A    | elmer   |  
  | A    | daffy   |  
  | A    | bugs    |  
  | B    | dawg    |  
  | A    | foghorn |  
  | B    | speedy  |  
  | A    | goofy   |  
  | A    | marvin  |  
  | B    | pepe    |  
  | C    | petunia |  
  | C    | porky   |  
  +------+---------  
  </code></pre>
  
  <p>I want to find or write a function to return a DataFrame that I would return in MySQL using the following:</p>
  
  <pre><code>SELECT
    team,
    GROUP_CONCAT(user)
  FROM
    df
  GROUP BY
    team
  </code></pre>
  
  <p>for the following result:</p>
  
  <pre><code>+------+---------------------------------------+  
  | team | group_concat(user)                    |  
  +------+---------------------------------------+  
  | A    | elmer,daffy,bugs,foghorn,goofy,marvin |  
  | B    | dawg,speedy,pepe                      |  
  | C    | petunia,porky                         |  
  +------+---------------------------------------+  
  </code></pre>
  
  <p>I can think of nasty ways to do this by iterating over rows and adding to a dictionary, but there's got to be a better way.</p>