Introducing a feature like this would dilute Google+. As it stands, Google+ is a social network; it's primarily intended as an online place to connect to people you know in real life. This feature would make it more of a broad "discussion center", where you talk about topics with people you don't know. That's a pretty fundamental paradigm shift.

In fact, the only similarity between Google+ and this feature is that both involve groups of people. That's about it. I don't think implementing this would improve Google+ at all; it may be a good idea for another site, and the groups thing may be a good way to go about it, but not Google+.