Hey Wormy,
If you can query the database server directly that's pretty easy to get. The query below will get you the information you need. If you are looking for something that project users or administrators can get to from within ALM/QC that is a bit more difficult.
The User's information is no longer stored in the project's USER table. Only the username is stored there. My thought on this would be to create an Excel report that grabs all the usernames in the user table and then in post processing use the site admin API to return all the email addresses for those users. Unfortunately I didn't have time to create a sample script for that, or to test it. So there you go. I hope this helps.

USE qcsiteadmin_db;

SELECT EMAIL, USER_NAME
FROM td.USERS U
WHERE EXISTS
(SELECT * FROM td.USERS_PROJECTS P
WHERE U.USER_ID = P.USER_ID)
AND EMAIL IS NOT NULL AND EMAIL <> ''
ORDER BY USER_NAME;