Sub groups without losing the context of the full group.

Different 'rooms'? You could keep the video tiles for all people on screen organized by room, then the individual chooses which one to join for audio. You could include a push-to-talk 'announce' button to send your audio to all rooms in addition to the normal open mic in the room you belong to at that time.

I picture a mosaic of webcam feeds that have border lines drawn to indicate group members.