What if a family member needs more space?
It doesn’t take many iOS devices recording video and making backups to overwhelm a 200GB plan. Everybody in your family group will be notified if you’re running low on space, and the family organiser can upgrade to a 2TB plan. That’s worth doing, because once you hit the limit of your storage then photos no longer upload to iCloud Photo Library, files no longer sync to iCloud Drive, and iOS devices will no longer back up to iCloud until you either free up space or add more.
Family members using a free 5GB plan are added to the family plan automatically. Those who pay can decide to stick with it or use the family’s quota, but can’t have both. If they decide to use your space they’ll get a refund for the unused portion of their own data plan.