iCloud storage join family sharing

Set up Family Sharing - Apple Support

On your iPhone or iPad

1. Open the Settings app, then tap your name.
2. Tap Invitations.

To accept an invitation, follow the onscreen instructions. When you join a family, you might be asked to confirm your account information and to opt in to any features or services your family set up.

Make sure whoever invited you used the correct email / Apple ID address.

It should also send an email with a link.