What's the advantage of only letting in 100 people at a time instead of making it publicly available? Personally, I'm not inclined to give you my email address without at least some sample content.
By letting in 100 at a time, everybody tries the product spread over time. They add comments to things, which alerts the people who joined a week ago, to get them to come back, and to inform them the illusion that it's an active community.
Zurb seems to me to have more marketers than it does designers. :/