Do you have unusually strong opinions about Google+ and what Google should be doing with it? Then today is your lucky day. Google is seeking people to help test new features and give feedback on the direction of G+. Not all who apply will be accepted, though.
This program is more involved than a Play Store beta test. If you sign up for Google's program, you will be granted early access to new features on Google+. Yes, that means there will be new features on Google+, which is news to us. You'll also interact with the product team at Google to provide feedback. So, it sounds like someone at Google is getting serious about improving Google+, and not a moment too soon.
If you want to toss your hat in the ring, there's a form to fill out that asks for your profile link, info about how you use G+, and how long you've been a user. Google is looking for people who are regular posters on Google+, so that's... not most of you. If you are chosen, I imagine there will be some sort of NDA to sign or ritualistic blood pact in which you must take part.