I've had an invite for over a week now but every.single.time I try to get on it tells me it's at capacity. I don't understand!! 
Just keep trying. They turn the invite access on and off through out the day (guessing so they can add/divert severs around accordingly) Most of the people Ive invited have been able to log in within an hour of receiving theirs..
I just added you, Chary. It seems to be a bit different than FB where it seems like you can be in someone else's circle without them being in yours? I am not sure how you request to follow someone's feed or if it happens automatically if they add you. I do like the concentric circles of privacy though.
Think of it like email being 'pushed' to your phone. If someone is following you but you arent following them, theyll appear in your 'incoming' stream If youre both following each other, status updates, links, etc will appear in your 'Stream'
You can search for someone, and if theyre there, send a request by clicking on their names and adding them to a circle.
Got it! Thank you!
Here's a question: How do you move someone from one circle to another? A friend's niece added me yesterday and I plopped her in my Friends circle. But then, on reflection, I thought perhaps she belonged more in my Acquaintances circle (she's not a grown up). I dragged and dropped her profile pic and she got added to Acquaintances, but she's still also in Friends. How can I get rid of her there?
As NoseOverTail said, go to the circle, click on the person you want to move and drag and drop into the desired circle they should be in.