I’ve joined LeanTalk a month ago. It’s a community where a specific topic about lean startups is discussed for 24 hours only, like “How do you know what a good metric value is?”. New topics are suggested and up-voted by the members. To get in you have to apply. There’s some really insightful advices there from time to time.

Note: The lean theme is becoming a bit hyped. There’s now services, consultants and everything dedicated to lean startups. But I believe the underlying principles are still important, like costumer development and measuring every assumption.