This support article tells you how to add a family member to your subscriptions from Apple --> Add a family member to your shared subscriptions - Apple Support Eligible subscriptions are automatically shared with your family members. This article tells you what types of content you can share with your Family Sharing group —> What types of content can I share with my family? - Apple Support
If you already set up Family Sharing
After you set up Family Sharing, eligible subscriptions are automatically shared with your family members. If your family members can't access a subscription:
1. On your iPhone or iPad, open the Settings app, then tap your name.
2. Tap Subscriptions.
3. Scroll down and tap Manage Family Access.
4. Turn on Share Eligible Subscriptions.
If you haven't set up Family Sharing
1. On your iPhone or iPad, open the Settings app.
2. Tap your name, tap Family, then tap Continue.
3. Follow the onscreen instructions to set up your Family Sharing group and invite your family members.
In turn the family members need to follow the joining instructions in How to set up Family Sharing on your iPhone, iPad, or Mac - Apple Support
Basically the family member needs to:
1. Open the Settings app, then tap your name.
2. Tap Invitations.
3. Follow the onscreen instructions.
If they are already members of the group then as it says about, they should automatically have the subscription shared with them.