Was in Outland the other day farming Thrallmar rep, and got bored. Flew around Hellfire P for a bit and found a few alliance players (lvl 61-68). Decided to follow them around and help kill mobs. They were hesitant at first (like, is this guy gonna gank me, or what?), but I think they got the point.
After a bit of killing stuff, I gave them a /bye and flew away.
I do that a lot although I don't stick around long usually because I don't want to disturb their natural environment. Also getting to comfortable around Horde can be dangerous for them.