title: nowjs group leave event called multiple times
body: <p>I am using now.js groups and want to listen for when a user leaves a group so I can perform some tidying up if the group is empty. I find that the leave event is called multiple times for each time a user has been in a group, i.e. if I join a group and close my browser 3 times on the 3rd time leave will be called 3 times.</p>
  
  <pre><code>chatserver.prototype.joinGroup = function(groupId) {
     //user joins group
     var group = this.nowjs.getGroup(doc_id);
     group.addUser(that.user.clientId);
  
     //want to know when a user leaves a group
     group.on('leave', function() {
         console.log("user left group:"+groupId)
         //do group clean up
     });
  };
  </code></pre>
  
  <p>Am I doing something wrong with scoping?</p>