>we would like to unlock free months to users who bring us new users through sponsorship. But since Apple manages payments, we can not do that
Switch to a non-renewing subscription IAP. But you must enable this 'unlock' system solely within the app, perhaps using CloudKit to transfer some sort of 'thank you credit' from one user to another. Apple needs some way of protecting itself from a user selling these 'free months' outside of IAP.
3.1.1 In-App Purchase:
If you want to unlock features or functionality within your app...... you must use in-app purchase. Apps may not use their own mechanisms to unlock content or functionality, such as license keys, augmented reality markers, QR codes, etc......