I need to add my card to family share plan

This document explains how to 'Change, add, [review,] or remove Apple ID payment methods' on an Apple mobile device, a Mac, or a PC; click here --> https://support.apple.com/HT201266

- If you are updating an old card and have ongoing subscriptions you may need to first add the new card and make it the primary payment method before deleting the old card.

Note that only the Family Sharing organizer can pay for something with a card. The payment method for Family Sharing is automatic:

1. If anybody in the family buys or subscribes to something, Apple first attempts to charge the item to that individual's Apple ID balance, if any.

2. If a family member does not have enough personal balance, any excess will be charged to the Family Organizer's primary payment method (usually a card of some kind). The Organizer's personal balance will not be used for purchases made by other family members. If it cannot bill the primary payment method then it will just work its way down the Organizer's list of payment methods until it finds one it can.

- "Some purchases, including gifts, can't be billed to Apple ID balance and will be charged to the family organizer's payment method." "Some subscriptions might not be charged to Apple ID balance."

Ref:

- Family purchases and payments - https://support.apple.com/HT201079

- How App Store and iTunes Store purchases are billed - https://support.apple.com/HT201359

- Check your Apple ID balance - https://support.apple.com/HT202359