He actually save about 3% by having you draft behind him. You would have disrupted the slight vacuum he created behind him. You both benefited. And that's the great thing about drafting.

As far as etiquette goes, as long as the person you're drafting is okay with it, and you find out by asking, then it's fine.

We had a situation yesterday where my guys (eight of us) were in a line moving at a decent clip on a road that's used by lots of cyclists, some slow some fast. We passed a whole bunch of groups as well as solo riders. At some point during a long stretch between lights I noticed that there were only 5 of us, plus some other guy. Apparently, as I found out later on, he cut into our line in front of three of our riders. I'd have said something right away, if I'd noticed it right away, but by this time 3 of our group had dropped back. Anyway, the dude stayed on our wheel for another couple of km before we hit a light.

Personally, I don't want some random stranger tagging on to our group ride. I don't know the dude, and so I don't trust him. I don't want to ride with someone I don't trust. Period.

Those of you who are comfortable with drafting random strangers, you're kind of playing with fire. Saving a few watts isn't worth the risk in this case.