As someone who is constantly baffled at many UX choices made by the makers of nearly every piece of software I use, I like the idea a lot. I'm looking forward to using the service.

And I do have a small couple critique of an aspect of your website. ;)

If you could get rid of the two input boxes after I've submitted my invitation request, that would make it more clear that the submission was successful. After I hit Submit, I did not immediately see the message below. If the two boxes were gone, I would have immediately seen that message.