If your cousin signed up as a Guest, she would have been greeted by one of our Greeters. Then, to complete her membership, she would need to answer some questions to become a Family Member. She then would have another message from a Greeter welcoming her and sending the WikiTree how-to link. She can access the path to membership from her Navigation page or her Profile page.
If she is already a member and you want to add her to the Trusted List to any of the profiles you manage here is the link about how to do that: https://www.wikitree.com/wiki/Help:Trusted_List
I hope this answers your question; if not, please let us know.