If a group catches you from behind it means they're generally faster riders, ans should move on in due course. If they don't, then hold your line and ease off slightly dropping 1-2mph off your speed, until they're clear.
By the same token, if you come up to a group, swing left and pass them and move on.
There's no harm in friendly greetings in the process, but telling people what they should or shouldn't be doing doesn't make anybody's day nicer.
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WARNING, I'm from New York. Thin skinned people should maintain safe distance.