That would be possible. It shouldn't be too difficult to use if you use GT modules. Take a look at GT::Mail, or you can always just use 'standard' mail modules. You'll want to use the GT modules to add the users though. I'd probably use their email as their username (there will probably be lots of duplicates if you use their user part of the email), and they would be able to change it later if they wanted to.
Adrian
Adrian