It's been a while, but as I recall, when it started it involved a proxy to inject the client code into pages, and a hosts file mapping that pointed an invalid FQDN ___._ to the backend, whose IP you had to get from someone who already knew it. Then, once you were in, there was also a karma system which limited the capabilities of low-scored users, and karma was hard to earn.
All told, it gave a strong impression of being less a web annotation system, and more a secret club for soi-disant cool kids. I didn't stay in long; a couple of days, no more.