It's not really closed, it's more like an "open beta" that requires registration first. I signed up just out of curiosity and submitted a bug report (which was a known issue, d'oh) and it never felt "closed".

You can sign up here https://launchpad.net/~ubuntu-typeface-interest after which you will be given access to the "private" https://launchpad.net/~ubuntu-font-beta-testing where you can download the beta.

edit: oh, it says all that at the top of TFA.