How do I re-add my daughter to Family Sharing after she was removed?

I just got an email from Apple saying my daughter "is no longer participating in Family Sharing". I dont know why. She didnt do anything. She is already over 13. But, her birthday is in October. She said she didnt changer her pw or anything. How do I add her back? I try to do it in my Settings>Family Sharing. But, it only allows me to create a new user.

[Re-Titled by Moderator]