You can share the app with a friend by clicking on the gift box icon on the upper left corner of the homepage of the app.
When you do, this an option to share the app with a friend will open up.
When you share the app, your friend will receive a gift code.
He will need to enter this gift code in the "Use Promotion code" section in the settings, which can be found in your app settings.
Once he does that you will both receive your 2 week Premium membership.