This (Dezign) Data Model shows Users, Registration, Communities and Collaboration.

A Design Comment :

There should be a check for Users registering
more than once with the same email address.

This could be done by a Trigger when the record is created, or after the fact,
(perhaps in a nightly batch run), to check for duplicates, using this
 SQL :-
   SELECT email_address,COUNT(*)
   FROM   users
   GROUP  BY email_address
   HAVING COUNT(*)>1;

 A surrogate (Auto-Increment) key would be used as the Primary Key, and perhaps
 a unique Index on the email_address field.