Sorry, my question was unclear, I was asking whether they will be using GitHub (or another publicly visible place) for their daily development or is this just where released versions will be pushed?

No, my answer was unclear :)

We will be doing all Light Table development in the open. We will be accepting pull requests (actually, I think we already have) although we require a CA (https://github.com/LightTable/LightTable/blob/master/CONTRIB...).

Incredible! Thanks and congratulations!