The portrait orientation is better suited to some meetings, like a fitness class or when all participants are using portrait sources (e.g., cellphones, tablets).
Using a cellphone or tablet in the portrait orientation is ideal for a fitness class. A camera in the landscape orientation needs to be too far away to capture the head and feet of the participant.
In the group view participants with a portrait source are shown centered in a large landscape-oriented box, even on a device held in the portrait orientation. Only one third of the landscape-oriented box is showing anything beneficial; the remaining two thirds is an empty black border.
Showing all the portrait sources in portrait-oriented boxes might allow up to three times as many users on the same line.