I found, however, that it wasn't immediately obvious how to add friends to a circle after creating one. On the other hand, it didn't take long to discover I had to click through to Manage Circles and then click on the circle I'd like to add people too. The process took a maximum of 30 seconds but this could be a lot quicker.
I have an idea though. What about having a button to edit the circle I'm currently in? I think that'd make it much, much easier. Another idea would be a small edit button when you hover over a circle when switching between them? This seems like it would be a little easier to fit in with the interface but would make it less obvious and could make that portion of the interface less usable.
I've put together a quick 'n dirty example of the first idea for you:
http://www.jameswdunne.com/example.jpg
I'm not really a designer though so I reckon something better could be done with more thought and skill (and source files rather than photochopping PNGs!)