Great question — answering as the UphillTech team:
Our app just got rejected yesterday regarding this kind of functionality. We had an application which includes yearly subscriptions to access premium features. We also had a way for users to refer 5 friends to get a free access for life to our services. Apple rejected the app stating that this was against their rules for IAP. Specifically, they referred us to the section 11.1 of the guideline which states : Apps that unlock or enable additional features or functionality with mechanisms other than the App Store will be rejected
Full answer by droussel: https://stackoverflow.stackexchange.com/a/17598251 (CC BY-SA 3.0).
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