I don't know, I've never really minded if people draft me unannounced. I tend not to do it myself, because I know some people are unnerved by it. If I come up behind someone who is travelling at almost my speed I'll go past them, but not accelerate. If they choose to hop on the back, they're welcome. From time to time this turns into a mini-break, with us each taking pulls in turn for as long as we're together. On these occasions it's rare for any words to be exchanged other than a "thanks" as we go out separate ways.