I've never had someone come up behind me and draft, but I wouldn't mind if they did. If they're a weaker rider, I don't mind to pull them along, if they're as strong or stronger than me, I would appreciate a break in return, but if they didn't, no big deal, that just means a better workout anyway. In my group rides, we all form a line and take turns pulling, we go about 30 mph, it's nice to be in the back going that fast, feeling like you can go faster! But pulling at that pace can get to be hard work, so we change who's pulling pretty frequently.