Yes, sign-ups are still open and you are more than welcome to sign up. The more the merrier!
While many people may already have betas in mind, we will make an attempt to find betas for those who are in need of one. We'll likely make a post, after assignments have been sent out, asking for beta volunteers - and if that doesn't work out for you and, at the end of all things, you're still looking for a beta, just send us an e-mail and we'll do our best to find you one.
I hope that helps. If you have any other questions, please feel free to either ask them here or e-mail us!