"Hey, how are you?" is good for me. I used to ride 100 miles every Friday. My 100 mile pace is not anywhere close to group ride pace, obviously. I'd get passed by folks I knew I could rip the legs from, and they would say things like "nice effort" or "looking good!" and it would drive me mad. Maybe because I'm not 130lbs like some of them, but I definitely got the feeling they were saying those things because they thought I was not as strong. I'm not proud to say I did speed up and drop people like that more than once. To me, it's in the same ballpark as putting your hand on someone's back to help them up a hill. You never know how fit someone is or how they might interpret something (rightly or wrongly).