In short, you can’t.
I’ve had another email via the contact form saying “I’ve tried to register, but I can’t, please register me” or somesuch content… of course the notion is patently ridiculous, since I have not activated any site registration code and have no intention of doing so…
Also when the request comes from someone who does not participate in any of the post comments, it becomes more ridiculous. So my assumption is that this might be the latest in an attempt to gain credibility on blogs running wordpress because once you are registered, you can post anything you like (spam, spam, spam, spam). If you are really good, you could hack the entire site.
Either way… sorry… registration has not ever been active here, and won’t be. You can ask, but don’t be offended if I refuse.