And as I added people to a circle, a bar at the bottom of the screen suggested that I could send them an invite so that they could also join Google Plus. Which was good, because not many of my contacts were actually on Google's Social Network.
The Demo of Google Plus suggested that it was only possible to add one friend to one circle, but I'm very glad to find out that this was just because it was a simplified demo. One can actually add a person to as many cirlces as one wants, which is a big plus - and also kind of obvious.
It seems to take about 10 to 15 minutes for an invite to be delivered, at least that's what I experienced.
What's interesting is that the Google Blog doesn't suggest anything yet about the sudden openness of their social network. Nor did I read anything else about it anywhere else (didn't search a lot either), or did I get an actual invite email. True; some of my friends do have an account and I do appear in some circles - but no invite email.
Anyway, let's see how open it now is. For those who don't have an invite yet, enjoy the screenshot below. Those who still remember Gmail invite from way back when might recognize the big similarties. Go go consistency!
Update: I managed to send out an invite to my Apps domain email addres, but when I'm logged in with that one I still can't even open the information page about Google Plus - due to Profiles not being available to Google Apps accounts.