Remember that if you're riding someone's wheel (especially on windy flats) you might think you could just pass the person ahead, but then find yourself in the headwind and see your speed much reduced, causing frustration for the person who was pulling to begin with.

Common sense is good.

In some groups, the leader of the ride stays in front of the group for certain parts of the ride. You might want to inquire. This being said, usually on hills it's each to their own...