DhU - Wiki
DhU stands for Dharma Underground, which was the original, members-only site founded a few years ago by Daniel Ingram so that people who wanted to talk about high-level practice could have a safe place to do so on the web.
The DhU was originally a wet-paint site, but that site has had no activity in a large number of months.
There is a version of the DhU here on this Liferay platform, but, as the spirit of open dharma has generally caught on, it gets little traffic, and is generally used by a few who haven't come out of the dharma closet yet to discuss things they are not comfortable discussing in public yet.
If you want to be a member, contact Daniel Ingram.