I would try it like this:
- set up a collection with a unique ref to every user, either being userid or email
- set up a signin/join page that needs Membership
- this will trigger the login/join dialog
- check the db if this user exists
- if so, redirect him to page x
- if not, redirect to page y (or .show() all hidden elements on that very page if you don´t want a redirect)
- on page y, show a form with anything you want them to fill out, write it to the db with either his userid or email
- next time, from 3 on he will go to page x because he already exists