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.
Hangouts is integrated with Gmail & Google+, does not require you to download desktop software, and all chats can take place directly in your browser, significantly reducing the barrier to entry for a new user.
Google offers all features of Hangouts completely free on iOS, Android and web. There are business, education and government solutions that increase the participant count to for video calls to 15 and offers support.
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.