upgrading storage to a family plan

The organizer of family sharing would need to subscribe to either a 200Gb or a 2Tb icloud plan. They they would send invites to family members to share that plan. If the family member says yes, then their own iCloud subscription plan (if they have one) will be cancelled and a pro-rated refund applied to their AppleID. They would access their content in the shared space using their AppleID.

Share an iCloud storage plan with your family - Apple Support - "With Family Sharing, your family can share a single 200GB or 2TB storage plan" https://support.apple.com/HT208147

All the steps are outlined in the article I link. Please read it to get a complete answer with Apple's own instructions.