some people seem to be quite unaware that they're wibbling in the road and at there's a car hanging back and waiting politely to pass. "Car up" can be pointed.
Assuming they're responsible adults, and with all faculties, I take the view that that is their problem, and the motorist's.
But that's utterly not the group-ride (as in a tight wheel-following bunch) ethic.
It might not be wibbling, it might be someone moving out to pass, move up, avoid an obstacle, or simply follow the wheels of those in front who might be 10 metres further up and unaware that there is a car behind.
It's not always possible to hear a car behind - that's a ridiculous assumption. They might have come up fairly slowly on a downhill section, or as another car went past. And if you are twenty riders up in a bunch then you can't all be looking over your shoulder all the time, even if you could see through everyone.
On a group ride there is an element of collective thought and action which is for the overall safety and benefit of the group. The physical nature of the group means that normal individual senses and reactions are compromised.
There's nothing worse than going out with people who don't give signals or relay information - the only excuse is if they're new, in which case they should be hanging at the back and picking it up.
If people don't like it, well that's why God invented audax.