Please understand that the app (and it is a 'single page webapp') is quite sophisticated, and while we think people will love it, communicating the ideas & concepts behind the app can be a little tricky, so we prefer at this stage to give accounts to people after they've had a demo or similar introduction.
It's not that the app is hard to use (quite the opposite) but that understanding what it is and what it does for you are not always immediately apparent.
Once we get the introductory materials sorted, we'll be more than happy to hand out more accounts.
So if you have anything further to go with your request (eg you've met one of us and we said we'd send you an invite, or we gave you a demo somewhere, or you have some other good reason why you should get an invite early), feel free to add a short message.
If you haven't already done so, it's well worth trying the Tour...
Or if you have an invitation code, visit www.mysparebrain.com/invite where, after you login with a Google Account, you can use the code to create an account immediately.