The easiest way to change this is to work on the scripts in /register/, to use a new column, which you need to create, in the parties table to represent the login id of the user. If you do that, you might also be able to relax the unique constraint on the email column of that table. You should probably still require an email address for account creation if that is possible.
If you don't ask for any email at all, you're in trouble when someone forgets his/her password, or wants to sign up for alerts on a bboard, etc.
But if you never want the email of your users, you could just ask for a user id and translate that into a "mailto:userid@yourdomain.com"; email address on your own domain.
