There is a list of all your circles on the left. Clicking on any circle will only show you posts from the people in those circles. As is said in the article, p

There is a list of all your circles on the left. Clicking on any circle will only show you posts from the people in those circles. As is said in the article, posts from people who added you but you didn’t add won’t show up in your stream but you can get to them easily if you want to. There are ways to improve this. At the moment you can only display one circle at a time. It would be nice if you could select more than one circle at once. Also, Google+ defaults to showing you all circles when you load it up and there is no way to change that. It might be nice to be able to only display posts from people in certain circles by default. But I think that’s some fairly capable filtering, at least if you manage your circles right.