Just announced by Google: “Anyone can sign up for Google+—no invitation required.”
Google+ has been considered in “field trial” since it was launched about 12 weeks ago. Many features have been added since then, and today they announced a bunch of additions which we’ve covered here: Google+ Hangouts Overhauled with Mobile, On Air, Doc Integration, and More.
In addition to those announcements, Google is now moving their social network to beta status and opening it to everyone: no invitation required. Just announced on the Official Google Blog is that anyone can join the Google+ project now by visiting google.com/+.