With iOS 14 Apple introduced the ability to enable Family Sharing for in-app purchases (previously it was just for paid apps). Family Sharing means that family members can share access to an in-app subscription (or single issue) without having to use the same App Store account. 

You need to manually opt in to Family Sharing for any in-app purchases you'd like to enable it for, in App Store Connect. It's nice and simple, Apple outline the steps in their doc here: https://help.apple.com/app-store-connect/#/dev45b03fab9

Note that users will need to be on iOS 14 or higher in order to benefit from it.