Hmmm, the bit which sounds rather dangerous is:
With some core joomla editing (tricky but it works) it's possible to register all users, even users with a non-unique email address.
Joomla, and all 3rd party extensions, have been written from day one with the assumption that email addresses for users are unique. The potential for unforeseen and subtle problems by breaking that Golden Rule could be huge.
What you *should* be doing, instead of adding users with the same email address, is something along the lines of what JFQ said, which is to use some other 3rd party extension, specifically designed to be a "mass mailer", and using mailing lists to handle who gets what mail.
The problem with "mass mailing" is that it is a VERY specialized task. While Fabrik does have built in mailing capability using a variety of plugins (list, form and cron), we are NOT a "mass mailer". Our mailing plugins are designed to handle relatively small amounts of recipients, measured in dozens, rather than hundreds or thousands.
When I say we are NOT a mass mailer, what I mean is that all we do is fire off a "best effort" send of the mail. We just hand the mail to J!'s built in sendMail() helper, which then uses whatever you have set J! up to use as your mailer. We provide zero features like checking to see if email got sent, or if it got bounced back, and handling disabling of addresses that consistently either don't send, or bounce back.
Those features are important, because not managing your mailing lists properly can very quickly get your site listed on any number of email RBL's (real time blacklists), on the assumption that you are a spammer, as typically they are the only "mass mailers" that keep trying to send to non-existent or bouncing emails.
The consequence of getting on RBL's are basically death to any mass mailing system.
We also don't provide any sort of built in "opt out" facilities, which are typically required for any commercial "mass mailing" system, so you'd have to roll those yourself.
This is something which we've devoted some thought to over the years, and I currently have some ideas about integrating Fabrik with MailChimp via their new Mandrill API. But that isn't going to happen "real soon", it's something I'll look at adding once we have released 3.1, and are free of the nightmare of having to maintain two huge code trees for J! 2.5 and 3.0, when we can deprecate 3.0, and have just one 3.1 code base that works on both J! 2.5 and 3.0.
Until then, if you want to use Fabrik, I'd look at following JFQ's suggestions.
-- hugh