title: Pandas DataFrame to drop rows in the groupby
body: <p>I have a DataFrame with three columns <code>Date</code>, <code>Advertiser</code> and ID. I grouped the data firsts to see if volumns of some Advertisers are too small (For example when <code>count()</code> less than 500). And then I want to drop those rows in the group table.</p>
  
  <pre><code>df.groupby(['Date','Advertiser']).ID.count()
  </code></pre>
  
  <p>The result likes this: </p>
  
  <pre><code> Date         Advertiser
   2016-01        A             50000
                  B               50
                  C              4000
                  D             24000
   2016-02        A              6800
                  B              7800
                  C               123
   2016-03        B              1111
                  E              8600
                  F               500
  </code></pre>
  
  <p>I want a result to be this: </p>
  
  <pre><code> Date         Advertiser
   2016-01        A             50000
                  C              4000
                  D             24000
   2016-02        A              6800
                  B              7800
   2016-03        B              1111
                  E              8600
  </code></pre>
  
  <p>Followed up question: </p>
  
  <p>How about if I want to filter out the rows in groupby in term of the total <code>count()</code> in date category. For example, I want to <code>count()</code> for a date larger than 15000. The table I want likes this: </p>
  
  <pre><code>Date         Advertiser
   2016-01        A             50000
                  B               50
                  C              4000
                  D             24000
   2016-02        A              6800
                  B              7800
                  C               123
  </code></pre>