We have weekly calls with about 30 participants and right now the screen just shows a random 4 tiles and often times it is people that are dialed in via phone and it doesn't rotate. I imagine for video group calls there is a way to see who is talking like in Skype for Business?
It's going to take active audio. The people on the dial in probably don't have their lines muted and it's showing those up first. However, the video feed / who's talking should dynamically shuffle to the front when audio is detected in the call from that individual.