I have an alpha-geek friend who sent me an invite a month ago ... but I didn't get to it right away and when I clicked I got a generic come back later message.
Last weekend the floodgates opened -- other people had gotten in, so I tried again and got in. Now even I can send invites.
Basically it's competing with Facebook. It gives me more control over who sees what... and there isn't the crappy clutter.
The "how to do circles" had "cycling geeks" as an example of the woman's circles... so I figure one of twho things happened: Enough people in Google's powerful mine-eery-word-you-type data stashes said "Bicycle" for it to be considered a cool hip thing to use as an example... or... they're already catering things to individuals and they know *I* am into cycling. I want a non-biker-babe to let me know...