iCloud Family Sharing is a feature from Apple that allows you to share eligible in-app purchases and subscriptions with your family members. Once you have activated family sharing on your iCloud account and invited your family members, they will be able to also use your Stoic Premium Subscription at no additional cost.
To enable family sharing for Stoic, select your name in the Settings app and tap Family Sharing. After you have invited your family members, you can enable Stoic Premium under Subscriptions to share it with your family members.
It might take a couple of hours for the subscription to be enabled for other family members and they might need to hit the Restore Purchase button on the paywall.
Family Sharing does not work for Lifetime option, only for subscriptions.
If you still cannot use Family Sharing for Stoic after following these steps, please contact the customer support.