After someone invites you to join their Family Sharing group, do one of the following:
If they invited you in person: On your device, click Allow to see a code. On the organiser’s device, enter the code.
If they sent you an invitation: Click the invitation you received from the family organiser using AirDrop, Mail or Messages, then click Join Family.
If you want to accept in System Settings: On your Mac, choose Apple menu Manage a Family Sharing group on Mac