The big button on your front page says "Take A Tour". "Sign Up" is tucked way down at the bottom and requires scrolling (it is also in the top menu bar sure, but it isn't as visible there). I knew I didn't want to take a tour since I figured I got the service from the blurb, but wasn't sure what else to do from the home page.

The largest button is not only the most visible activity, but it's also an implicit suggestion to your users. Do you want people to sign up, or do you want them to take a tour? Plant the idea in their mind with design.

Smaller notes: I have no idea how to generate that funky calendar I saw in the tour, or to tell the system that I've completed an assignment once created. Also - just to be picky - the header colors really don't work for me. Blue and green and another blue, and another blue and dark grey and light grey? Multiple borders and banners? If you aren't working with a designer, try keeping the site simple and iterate towards a more attractive site. Design that adds complexity works against the impression of simplicity and usability (it communicates money). I believe your logo against a simple white background would be more attractive to potential users than the ruckus up there now. Simpler design would also call more attention to the content you really want users to see.