Usually I make it very clear that I'm trying to run with the person -- I'll run right next to them, and usually say hi if they look over. The worst I've seen was when I was in highschool and ran 6:30 pace on some of my "easy" days (I didn't know better), and I go by three guys on the Central Park Drive in the summer. You can see one of them looks really pained to see me go by, and his friends all say, "Oh, you go ahead, we're holding you back," but the guy was clearly out of shape (read: obese) himself so made some excuses and they stuck together.