You will find the "Challenge your friends" option at the bottom left-hand corner of the main screen of the game, below the "Play" button. Once you select it, a pop-up window will appear with different social application options that you can use to send an invitation to your friends.
Once your friend receives the message and downloads the application, he or she will appear as an available opponent every time you want to start a new game.
If your account is linked to Facebook, you can invite your friends when starting a new game from the "Play" button on your home screen. After selecting your desired game mode, you will be redirected to the "Select opponent" screen where you will find your Facebook friends who currently have an account in the game and, also, the option to invite friends who are not yet in the game.
For more information on how to link your account to the social network, we suggest you visit the article How do I link/unlink a Facebook profile to my account?