As of v2.6.0 you will see that adding friends has changed! You can now access the friends screen from more screens too.
The fastest way to invite a friend is to click the Green add friends smiley icon, followed by sharing a link to your profile via messaging or social media applications.
Alternately you can also copy your Player ID by clicking on it, then send/share it with a friend so they can manually input that on the add friend screen.