How do you share added storage with family members. Can they access each other’s storage?

See the information in Share iCloud+ with your family - Apple Support including the following:

When you share an iCloud+ subscription, everyone in the family group uses their own account. That way your photos, documents, and other information stay private, even though you're sharing access to the service. You simply share the features and space available in iCloud+ with your family members, so you only have one plan to manage.