The small issue here, it ask for email, but no username, as we aware that we can have different username and email address in normal Salesforce login. When you register with your existing email address, it will say "an account with this email address is already in use."
The simple solution is to create new email address in Gmail.com or Yahoo.com, but for many people (including me), it will be a bit troublesome to write that email address to remember it, although you can use tool like Evernote to remember it.
In Gmail, all characters anything after and including the "+" sign will be ignored.
Example - these are all the same:
All samples above will reach email firstname.lastname@example.org (note this is just a sample, not my real email address).
So, we can make use of this for to register for Salesforce Pre-release, so register with email@example.com will allow you to register without creating new email address :)