Interested in an invite too...
Circles looks interesting but Facebook already has similar features, of course, it took me a while to figure out how they worked. Here, ill explain:
Circles looks just like groups in Facebook. How is it different? You can make private groups in Facebook and invite friends, like a private message board.
If you want to hide posts from people you can do that too. On the edit friends page you can make lists and add select friends to it. Then when you make a post click the lock icon, click custom from the drop down and type the group name in the field for only visible to these people. Voila, your post is now hidden to everyone except the people in the list you gave permission to it.