Alongside the "Install Web Reader" link there's a "Start Learning" link which will take you into the web-app and allow you to sign up. I figured "Start Learning" would be more inviting than "Sign Up" but this is something it might be worth AB testing.

I know, but almost every site has sign-in and sign-up link at the home page. I am a developer, but was confused how to sign up. I thought it was some kind of invitation model... I do not see any problem with putting a sign up link on the homepage. The more ways for your to sign up, the better.