BigBlueButton

BigBlueButton is a web conferencing system, introduced now in Mattermost.

Not available from phone. Please use your computer, either Web or App

It's very easy to use! just access to any Mattermost Channel or Direct Messages you want to start a meeting with and click this icon on the upper right corner:

The meeting room is very user friendly and displays a welcome board. But if you want to know more and take all the potential, just visit their official Tutorial Videos.