On the main screen of the game, you will find the "Invite" button on the left side under the "Play" option. Once you press the button, a pop-up window will open with different social applications through which you can invite your friends to the game.
Furthermore, by selecting the "Play" button, you will be redirected to a new screen from which you can select your opponent. There, represented by a pink button below the search bar, you will also find the "Invite" option.
Once your friend receives the message and downloads the application, it will already appear as an available friend every time you want to start a new game.
If you have linked your account to a Facebook profile, once you select "Play" from the main screen, you will be able to see the friends who have the application on their devices. By clicking on the photo of your friends, you can start a new 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?