You can add friends by pressing the yellow "weekly stats" button and navigating with the green buttons to the "Friends" page. Press "Add Friends" and write your friends "friend code" and press "search". You can find your friend code under the search bar that you can also share with your friends.

NOTE. You and your friend both need to add each other as a friend in order it to show on both of your games'. Friend code can also be found in the settings menu next to your userid.