You will find the option "Invite your friends" in the lower left corner of the main screen, below the "Play" button. Once you select it, a pop-up window will open with the different social application options that you can use to send an invitation to your friends.
When your friend receives the message and downloads the application, it will appear as an available friend every time you want to start a new game.
If your account is linked to Facebook, you can invite your friends to start a new game by clicking on the "Play" button on your home screen. After selecting the game mode you want, you will be redirected to the "Choose opponent" screen where you will find your Facebook friends who currently have an account in the game.
For more information on linking your account to the social network, we suggest you visit the article: How can I link/unlink a Facebook profile to my account?