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.
EFF gave Viber's security 1 out of 7 points in November, 2014. It deducted points because the provider is capable of reading encrypted messages, there's no way to verify contacts identities, past communications are not secure if encryption keys are stolen, the code is not open to independe...
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.
At $6.67 per user / month, Slack is significantly more expensive than the competition if you need features such as unlimited integrations or unlimited message storage. However, the free version of Slack includes unlimited users.