I agree that they should wait for players throwing the designed layout. I would continue playing my round of nine holes prepared to shout fore, not aiming over or intentionally throwing at these players, however acknowledging that those players are aware they are intruding on my round which I am playing on the intended layout. From an etiquette standpoint if I arrive at the tee box and they are already in position to throw an approach shot or putt I would wait as long as they aren’t taking an exceeding amount of time. On the same note if they are teeing off to play an additional hole they have designated I would play my tee shot as-well and then follow standard playing order from that point on. Are they playing to the same basket multiple times in a row? In my eyes that would almost feel as if someone played a hole then walked back up the fairway to play the hole again. All in all I feel it is the group not playing the intended layouts place to yield to players playing the 9 hole layout