title: Pandas - Group by one column and aggregate other column to list
body: <p>I have a dataframe that has multiple entries for users. These users can also be assigned to multiple ID's.</p>
  <p>I would like to group by the users and then store a list of these ID's in another column as shown below:</p>
  <p>I'd like to go from this:</p>
  <pre><code>df1 = pd.DataFrame({'USER': ['BOB','STEVE','PAUL','KEITH','STEVE','STEVE','BOB'],'ID':[1,2,3,4,5,6,7]})
  </code></pre>
  <p><a href="https://i.stack.imgur.com/12Wdi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/12Wdi.png" alt="enter image description here" /></a></p>
  <p>To this. Only showing values if that user is attached to multiple ID's</p>
  <p><a href="https://i.stack.imgur.com/Mda9p.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Mda9p.png" alt="enter image description here" /></a></p>