On the main screen of the game, you will see the option "Invite your friends" in the lower left corner of it. Once you press the button, a pop-up window will open with different social applications through which you can send the invitation to your friends to download the game.
Once your friend receives the message and downloads the application, they will already 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 do I link/unlink a Facebook profile to my account?