Ok, I'm posting so I obviously managed it but there is no obvious sign up button. I got there by going to the sign in page and then finding that if you hover over "sign up" it's a clickable link. However it only shows as such when you hover over it (Firefox) A sign up button on the home page and in other places as well would be good.
We're adding a Sign Up link to the menu bar. You mention on the sign in page that you only realise that sign up was clickable when you hovered over it - does the red colour not show up sufficiently to distinguish it from the black text (given that red is clickable throughout the site)? If not we'll try to make it more distinguished.