title: Pandas dataframe: groupby one column, but concatenate and aggregate by others
body: <p>How do I turn the below input data (Pandas dataframe fed from Excel file):</p>
  
  <pre><code>ID      Category                    Speaker     Price
  334014  Real Estate Perspectives    Tom Smith   100
  334014  E&amp;E                         Tom Smith   200
  334014  Real Estate Perspectives    Janet Brown 100
  334014  E&amp;E                         Janet Brown 200
  </code></pre>
  
  <p>into this:</p>
  
  <pre><code>ID      Category                    Speaker                 Price
  334014  Real Estate Perspectives    Tom Smith, Janet Brown  100
  334014  E&amp;E                         Tom Smith, Janet Brown  200   
  </code></pre>
  
  <p>So basiscally I want to group by Category, concatenate the Speakers, but not aggregate Price.</p>
  
  <p>I tried different approaches with Pandas <code>dataframe.groupby()</code> and <code>.agg()</code>, but to no avail. Maybe there is simpler pure Python solution?</p>