Thanks to Siobhan for showing me how to chat to one single person at a time. Click "Add circles or people" then type in the name of the person you'd like to speak to directly via an initial message and comments beneath this. Only problem with it is that only they can see it, so no public conversations this way on Google+. Though I suppose that can be achieved through a "public" message. You can ensure that the person receives this "public" message by adding them individually (ie. typing their name in the box that says "add more people".