Sadly, Facebook discontinued support for in-game friend invites, that is why this option is unavailable.You can, however, see your friends who play Manor Matters if your game is linked to your Facebook account.

To find your friends in the game, go to the game's settings and log into your Facebook account. All of your Facebook friends who play Manor Matters as well will appear on your friends list in the game.

To get more friends in the game, you can add other players as friends on Facebook. Players are listed on the game's official page.