I used quickjoin to join a game for a private sale. While we were trading, another friend of mine joined in. I checked my quickjoin feature and it was not enabled. The new joining friend was not a mutual friend of the person with whom I was trading. He had to have quickjoined my character. I clicked apply and ok despite not changing the settings. The person I was trading with left the game and verified that my character was quickjoinable despite the box being unchecked in the social tab.