Also if there's anything really weird with a rider - saddle is way too high, missing (seriously, there was a guy that used to ride in the NY area that insisted that riding without a saddle or seat post was beneficial), bizarre bar or saddle angle, etc - it's usually because the rider rides by themselves all the time. In a group ride, even if it's not a club ride, they'd probably get feedback questioning the bizarro stuff.
Basically as a rider gets more fluent they become less noticeable in a field. The weird ones stand out because they do unusual stuff, stuff you don't expect.
Someone said to me "school of fish" yesterday. In a school of fish the one going the wrong way stands out. A skilled group rider won't seem out of place. The ones that disrupt the rhythm are usually doing something wrong.