You can also use the Share button by pressing Invite in the friends window to send an invitation to the game through social media or messengers.
It is also possible to add a random player from anywhere in the world to your friends list! To do this, press the "Find more friends" button. You'll see a list of people you can add to your friends list. This list can be updated once every 12 hours, but you can always spend rubies to update the list more quickly.