Ask HN: What's a good tool for private group communication?

I want something that I can use to communicate with just the friends I usually hang out with, for project groups, for dorms and even with my own family.

Email is the current solution but email's really disorganized and not fun. Google Wave is too complicated although I feel that it's close to the right solution. If you could build your own solution, what would it be? Do you know any existing solutions that are great for something like this?