On the Slate Star Codex announcement Scott (Yvain) said, "If you're reading this, you're invited", so all are welcome. There are usually one or more lurkers/people who don't really know anyone. No sign-up necessary.

There is no hard time limit, but it seems like previous ones have often lasted like three hours or more if I remember correctly. But you can leave (or arrive) at any time.

I hope to see you (and blumsha) there!