Yeah, there is no mutual accept/reject friend request function like on Facebook. When you add someone to a circle, you are allowing that person to see anything that you post to that circle. They don’t have to accept the request; they’re just in there.
However. If you are not in any of their circles, then your posts will show up in their “Incoming” feed, which is separate from the main stream. You can access the Incoming feed via a link on the lefthand side of the screen. Once the other person adds you to a circle (any circle at all), then posts you publish to the circle they are in will show up in their main stream.
Example: I add my college friend Mary to my Friends circle. I make a post and publish it to my Friends circle. Mary doesn’t have me in any of her circles yet, so my post goes into her Incoming feed. She checks her Incoming feed and sees my post there. “Ah ha,” she thinks, “that’s MsWhatsit. I know her from college.” She adds me to her College Friends circle.
As soon as I am in one of her circles, my Friends post will show up in her stream, instead of in the Incoming feed.
However, if I make another post and publish it only to my Acquaintances circle, then Mary will not see that post, because she’s not in my Acquaintances circle. She’ll only see the stuff that gets published to Friends. (Or “all circles” or “Public”.)
Yes. You can also select a checkbox that will email people without Google+ accounts every time you make a post to a circle you’ve added them to, but I would recommend against this because it can become spammy and annoying.