Hangouts works with up to 10 participants using video out of the box or up to 15 if you are a business, government or school. It determines who is the primary speaker at any given time, and the camera view automatically switches to that user.
With Google+ “Hangouts on Air”, you can broadcast any hangout to an unlimited audience via your Goolge+ Profile or simply a generic YouTube link. Moreover, any Hangout on Air (public or private) is automatically saved as YouTube video that you can keep for private reference or use in any way that y...
There is no need to install any software. You can easily use the app from your browser. You don't have to sign up, just start a room and you're good to go. You can then claim the room's URL to reserve it to yourself.
While Slack has no built-in video chat, it integrates with Hangouts and enable such functionality. Additionally, recent acquisition of Screenhero promises eventual built-in support for audio and video conferencing.