title: Pandas convert a column of list to dummies
body: <p>I have a dataframe where one column is a list of groups each of my users belongs to. Something like:</p>
  <pre><code>index groups  
  0     ['a','b','c']
  1     ['c']
  2     ['b','c','e']
  3     ['a','c']
  4     ['b','e']
  </code></pre>
  <p>And what I would like to do is create a series of dummy columns to identify which groups each user belongs to in order to run some analyses</p>
  <pre><code>index  a   b   c   d   e
  0      1   1   1   0   0
  1      0   0   1   0   0
  2      0   1   1   0   1
  3      1   0   1   0   0
  4      0   1   0   0   0
  
  
  pd.get_dummies(df['groups'])
  </code></pre>
  <p>won't work because that just returns a column for each different list in my column.</p>
  <p>The solution needs to be efficient as the dataframe will contain 500,000+ rows.</p>