Here is a list of both Google and Yahoo groups that have some focus on Lean and/or Agile Development. You will need to sign up with Google and Yahoo to join these groups. Most of these groups are very active and you will find it's a good resource for both newbies as well as seasoned agilists. Enjoy!
Agile Tangents (http://groups.google.com/group/agile-tangents) - Good overall discussions around all things Agile
Agile Project Management (http://finance.groups.yahoo.com/group/agileprojectmanagement/) - Focus on the management side of Agile projects
Test Driven Development (http://tech.groups.yahoo.com/group/testdrivendevelopment/) - Everything you want to understand about this practice also known as TDD
Scrum Development (http://groups.yahoo.com/group/scrumdevelopment/) - Emphasis on just Scrum brought to you by the Scrum Alliance
Lean Development (http://tech.groups.yahoo.com/group/leandevelopment/) - Focus on the content of the Lean Development books by the Poppendieck's
LeanAgileScrum (http://tech.groups.yahoo.com/group/leanagilescrum/) - A newer group started by Alan Shalloway from Net Objectives. Brings together Lean, Agile and Scrum discussions in one place and talks about how they work together.
If there are other community groups, please let me know by responding via comments. They can be other Google or Yahoo groups or other types of communities that focus on Agile or Lean.