Notice that Google+ has no 'messages' feature? No worries. Just make a post private between you and another person, then add a star with this extension. It'll be there for you, as if in your inbox.
This is a proof of concept on how a messaging/email system within Google Plus might work -- just like starring emails in Gmail.
The Google+ PowerApp lets you add a person to your circles right inside your emails. The app shows you a short summary of the particular user associated with that email, and lets you check off which circles you'd like them to be added to.