Whether your friend has already downloaded the game or not, the steps to follow are the same. In the friends tab, tap the invite friends button. You can then forward the link to your friend by copying and pasting or via an external application (email, text message, Facebook, WhatsApp, etc.).

As soon as your friend accepts the invitation, you will receive a notification and your friend will automatically be added. 

You can also request to add your previous opponents as friends in the game. Tap on the opponents tab and then on the add friend button for the player you would like to add. 

If you would like to remove the friend request, tap on the picture of the player to navigate to their profile. Here you will find the revoke friend request button.