Google+ Hangouts has been the standout feature of Google's social network and it didn't go unnoticed. Google realizes that it's one of the strengths of Google+ and it's something no one else offers at least not for free.
Which is why Hangouts is being pushed into every other Google product where it makes sense.
A while back Hangouts was integrated into Gmail enabling users to start a group video chat without needing to go to Google+.
Now, the feature has been built into Google Calendar by enabling users to attach a Hangout to events. When creating a new event users have the option of also adding a Hangout to it. This way you can schedule group chats in advance or use Hangouts to iron out any details.