Sadly, there's no option to send invitations or friend requests in game directly. You can, however, see your friends on Facebook who play Mystery Matters, too, if you link your games to your Facebook accounts and grant the game access to your friendlists.
To do so, go to the game settings and log into your Facebook account. Then, open your Facebook page in browser, go to Settings → Apps and Websites, find Mystery Matters and make sure that the friends list is shared. All of your Facebook friends who play Mystery Matters will appear on your friends list.
You can get more friends in game by adding other players as friends on Facebook. We recommend you visit dedicated fan groups where players look for friends to exchange gifts. After you send them friend requests, these players will appear in your game!
Please note: Access to Facebook services can be restricted in certain countries or regions. In such cases, we comply with local laws and regulations.