Short version: That camera button? It creates a topical Hangout, visible to those who see both you and the post. Nifty.
Quick tip: Hangouts are supposed to be on-the-fly, if-you're-available chats. Sometimes you'll come across something that you really want to chat about, and maybe invite some people who are seeing it and reacting to it to do the same.

So if you want to create a Hangout around a particular public post, or maybe something you've shared with a select circle or set of folks, look for the camera icon below the post. That instantly creates a hangout with the post as a topic, and everybody who has you in their circles can see that Hangout happening when they view the post.
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