In BI's built-in "Help", you can read the how to create a group or groups at "Cameras" => "Camera Groups"..
A camera can trigger a group as mentioned, again, in BI's "Help" under "Cameras" => "Triggering and Motion detection".
You open the "settings" of the camera that, when it's triggered, you want to also trigger a group, click on the "Trigger" tab and toward the bottom on the right check the box "Trigger camera groups" and select the desired group(s) to be triggered from the drop down.