According to an interview with Adam Driver the Knights of Ren existed before Kylo/Ben became a member. Maybe, I’m not too sure what to take from this statement.
http://www.starwarsnewsnet.com/2015/12/j-j-abrams-and-adam-driver-talk-about-kylo-ren.html
"So who are the Knights of Ren?"
"It was a group that existed before him, that he was a part of. Their place within everything is maybe more of a satellite group than I would say … This is really tricky."