Found 21st Jun 2016
"Refer a friend to our app, by sending a special someone your unique code in the Burger Pusher section of your app. And you'll both get a free side! Wow, what a great friend you are! (Sure, sure, you also get a free side when they do it, but you're really doing it for them, right?) Yup, it's our 'Burger Pusher' offer, designed to get you both eating free stuff. ". You need to update app to latest version for this feature.