title: pandas - groupby column with multiple values
body: <p>I want to display the users that have used a value. </p>
  
  <pre><code>import pandas as pd
  user = ['alice', 'bob', 'tim', 'alice']
  val = [['a','b','c'],['a'],['c','d'],['a','d']]
  df = pd.DataFrame({'user': user, 'val': val})
  
  user    val
  'alice'      [a, b, c]
  'bob'        [a]
  'tim'        [c, d]
  'alice'      [a, d]
  </code></pre>
  
  <p>Desired output:</p>
  
  <pre><code>val     users
  a      [alice,bob]
  b      [alice]
  c      [alice,tim]
  d      [alice,tim]
  </code></pre>
  
  <p>Any ideas?</p>