@Annerle0
Go to Listing Editor/Your Space
Scroll down to invite a Co-Host
Be sure your stepdaughter has created a profile on Airbnb and send the invite to the email she added to her Airbnb profile. It's best to do this on a desktop/laptop instead of the App on your phone. She should also use a computer to click the link to accept in her email in box. I have found trying to click the link in the email on your phone is "glitchy" and sometimes the link doesn't appear at all.
How to Add A Co Host
https://www.airbnb.com/help/article/1244
Per @Helen3, it's best to use the Airbnb Help Center to research how to accomplish things on the platform. Just click your profile photo/Help Center search the topic you are interested in. The link above explains how to add a Co Host.