code: function addPlayer(userId, callbacks) {
          let player = factory.addPlayer();
          if(!player) return null;
  
          self._callbacks.set(player, callbacks);
  
          // If the game didn't have an owner, it does now
          // This should only happen when the game is first created
          // and the initial player is added
          if(!owner)
              owner = player;
  
          self._users[userId] = { id: userId, player };
  
          return player;
      }
docs: callback is used to send messages to the players in the game