You can invite friends via a link through different social applications. To do this, from the "Friends" section (the two-person profile icon, in the bottom right corner of your home screen) you must tap on the button represented by a person, which you will see in the top right corner, above the search bar.
When you do so, different social apps you can use to invite your friend to the game will be displayed.
Once your friend receives and accepts the invitation, they will appear as a friend available to play every time you want to start a new game.