I have looked through our auth0-acul-js signup() documentation and did not find a way to pass additional data like the family_name, given_name, or company attributes to the signup() method.
This was justified by your experience of only seeing the email attribute when you tried to console.log the event.user object.
Since you are essentially creating a user on their behalf, you could consider using the Authentication API to create the user and pass in the family_name, given_name, and company attributes.
To bad, I would have liked to be able to pass custom data that way that then would be added as user_metadata, but ok, if that is not possible than we will create the user as we do today, by using the API you also suggest.