Goolge didn’t announce this formally about that new feature.
Now you can hangout with friends on youtube video directly from youtube, Watch with your friends. :)
Couple of days ago youtube live manager mentioned that future ability to hangouts from You Tube with live video streaming, and this is must be the first initiative of that.
It’s been possible to watch videos with friends since the beginning of Google+, through a more complicated manual process that must be initiated in Google+. But this new button allowing you to initiate hangouts from YouTube with a couple of clicks makes it considerably easier.