How to invite people by email or contact list to Google+
(July 3 note: this was written on Jun 29, second day of Google+ launch, before Google opened inviting ability and then shut it down again. This technique may or may not work now, as Google is trying to control growth. I am getting emails from around the world asking for Google+ invites, maybe because this post comes up high on Google search results. Last report is my invites are not going through now, perhaps of a long queue).
1. Create a post. I suggest something that explains that the post is being used to invite them to Google+, also explaining that invites are being limited. (They will get that post in an email with the first few words as the subject line)
2a. Inviting by suggested contact list: Create a circle with everyone you want to invite in your contact list (I created one called, creatively, "Invitees"). Add that circle under "Share"
2b. Inviting by email: Typing their names directly into the box under what you want to share, where it says "+Add more people."
3. Hit share.
Note, I've been informed by a Googler+ that invites are rate limited by hour. So if they can't click through that first time, they can try again later with same link. (You don't need to resend it)