“Family has other members” box when trying to join Family Sharing

Did you also disband your now unused Family Sharing group? If not it is still active and you are a member of that group. As you are a member of that group you are prevented from joining another group.

Apple has instructions on how to disband a group in Leave or remove a member from a Family Sharing group on iPhone

Disband a Family Sharing group

When the family organizer turns off Family Sharing, all family members are removed from the group at the same time. When a Family Sharing group is disbanded, all members immediately lose access to the shared content and subscriptions.

Note: If a child is in the Family Sharing group, the organizer must move them to another group or delete their Apple ID before disbanding the group. See the Apple Support article Move a child to another group using Family Sharing.

1. Go to Settings  > Family > [your name].
2. Tap Stop Using Family Sharing.