Google Plus circle and email

Go to Manage Account Information and add an additional address.

Although I didn't test it, I'm quite sure they do. If you add an e-mail address to your circles, that address would immediately be linked to that person's profile as soon as he signs up. Then just the same check happens when he tries to look at that post, he will thus be in your circles at that moment.

What I don't know is whether he will see your posts in his Stream if he logs in or whether he will only see posts made after the time he signs up.

I'd say, you probably got plenty of friends not on Google+ so plenty of possibilities to try it out, let us know!