title: How to add "join group" button on facebook?
body: <p>In my website, I would like to add "join group" button for my facebook group.
  I know this is possible to do with "Like", but can't find a way to add a button for joining a facebook group.</p>
  
  <p>Is it possible? Am I missing anything?</p>
  
  <p>Thanks</p>