Have taken the route: Admin>All Admin>User Tools Home>Manage User Access>Manage Experience Cloud Users
In the Admin Console, clicked Add User - the email was added, the correct property was assigned, the user as expected received a welcome email.
The user in question has been unable to log in, getting multiple error messages upon login with the 3 field prompt. Her attempt to login via 'experience cloud' link drives her right back to the 3 field prompt.
Upon inspection of Admin, I see her name show up within the property she was assigned. However outside of Admin Console within the User & Assets section of Analytics her username is not* found.
Outside of this she has attempted to reset her password. She receives confirmation she has reset successfully. Yet still the login error messages persist.
You mentioned "property", so I'm not sure if that's the same as what I mean by "product profile". See https://helpx.adobe.com/enterprise/admin-guide.html/enterprise/using/manage-product-profiles.ug.html.
If the user is assigned the correct Analytics product profile (the profile should have all the permissions), the user should login. There seems to be some configuration error. I'd recommend that you reach out to Adobe Client Care with the Org ID and the Adobe ID/Federated ID and the team will address it soon.