If I add someone from my Gmail contacts to a circle, and they haven't got a Google+ account, do they get an email saying I've added them on Google+?
No, they don't get a notification. However, when you share to that Circle, Google will ask if you want to send an email update to people not using Google +.
As of this post they do, in fact, receive an email, if you add them from within Gmail. I made the mistake of adding an important client to a Circle with an accidental click, and they received an invite to join Google+. That resulted in a very awkward phone conversation where they were letting me down easy about how they don't use Google+, and I was letting them down easy that I wasn't inviting them to my social network.
I wrote an extension to prevent this from happening again that disables the button: https://chrome.google.com/webstore/detail/disable-gmail-add-to-circ/nckhnfmcmenbmoabnfcmdiabconcbhdk