It's important to understand what content is shareable and what isn't with regards to Apple's Family Sharing program. Because Pocket Prep is considered a third-party subscription, our apps fall outside the scope of the program. In other words, your premium access is limited to your individual Pocket Prep account. We apologize for any inconvenience this may cause.
The following snippet was taken from Apple's support article on Family Sharing.
What you can't share:
Individual or student Apple Music subscriptions and other third-party subscriptions.
Items that are no longer available on the App Store and iTunes Store.
Items that you or another member of your family group have hidden.