title: Pandas dataframe, keeping values with the same index
body: <p>I guess the title is kinda confusing so let me show you first the data</p>
  
  <p><code>UserID                 appid           playtime
  0  76561193665450065     10                26
  1  76561193665450065     20                 0
  2  76561193665450065     30                 0
  3  76561193665450065     40                 0
  4  76561193666198401     420                 3
  5  76561193666198401    8190                64
  6  76561193666198401   72850              8717
  </code></p>
  
  <p>So i want to make some calculations but i need to take one UserID per time and check his appid and playtime.</p>
  
  <p>I need something like <code>df['UserID'][i]</code></p>
  
  <p>Due to request, the result that i want is </p>
  
  <p><code>UserID               appid           playtime
  1  76561193665450065     10                26
                           20                 0
                           30                 0
                           40                 0
  2  76561193665450446     40                48
                           110               20</code></p>
  
  <p>I dont know how many rows are in each UserID.
  I want to select UserID and see all the appid that he has.</p>