When spectating a group workout, the action bar at the bottom shows controls to enable and disable ERG mode, and increase or decrease the difficulty. The increase and decrease buttons are also shown about the workout details.
I am not sure if they actually affect the rider being watched, but probably better not to show them at all?
Also, the workout progress is not shown? The event I was spectating was nearly finished, but the workout details still showed as if the warmup hadn't been started.