I don't like that you show accounts for Twitter and Google and then just say they aren't available...I understand the reasoning behind it but just show the images and don't provide links. EDIT: On a 13" MBP the "Have an invite code" link is below the fold and was difficult to find at first.
I initially put that in there to measure (using mixpanel) how many people wanted those, to avoid overbuilding. Learned pretty quickly that people want a Twitter auth option. While we build it, wanted to at least acknowledge that they are on their way.
In general I think you've got a great idea with this product. Solid alpha/beta. Keep up the good work!