If they were Sith, they'd call themselves "Sith." Also, there'd probably be only 2 of 'em. And Kylo Ren would be called Darth Kylo or something.
The Knights of Ren, I think, are probably a small group of warriors that use the Dark Side of the Force, but are organized in a fashion that sort of "mocks" the Jedi Order or whatever the structure of Luke's revived Order was. Basically, they're anti-Jedi, not Sith.
There are other fictional universes where some of this apparently happens. For example, in the Dragonlance series of books, you originally had this order of knights called the Knights of Solamnia. They were instrumental in defeating the evil dragons and beating the dark goddess who sought to enslave their world several times. Eventually, the dark goddess creates her own order called the Knights of Takhesis, who are loyal to her, but who use similar organizational structures, training methods, etc. as the Knights of Solamnia. So, they basically become the evil version of the good organization.
I'm guessing -- just guessing, mind you -- that the Knights of Ren are something along these lines.
I'd further speculate that the overall approach to the Force is going to become a lot more nuanced than just "Light Side/Dark Side" and never the twain shall meet. My sense is that the Light/Dark split is going to be revealed as more philosophical than absolute. The Force is just...the Force. It can be light, it can be dark, but it's all the Force. The Light/Dark discussion makes it sound like there are two Forces, almost, and that they are in opposition to each other. But that has more to do with the people who use it, and how they use it than it does with the nature of the Force itself.
For me, this ties back to the whole "Prophecy of restoring balance," where some interpreted the prophecy to mean that Anakin would destroy the Sith. Technically, he did. But he also sired Luke, who in turn may have sought to (or may yet seek to) "restore balance" by doing away with notions of the Force itself being light on one side, and dark on the other, and looking more to how a Force user interacts with it.
The way I see it, the Jedi approached the Force from the perspective that you could never ever touch the Dark Side, or get too close to it, and that you had to be devoid of emotion to most effectively and safely use the Force. Luke sort of demonstrated that was wrong when he used the force offensively (e.g. choking the guards at Jabba's palace, mind-tricking Bib Fortuna to let him in to the palace, etc.), and he also demonstrated that you could walk away from the dark path. I think that's proof that a good person can use what others might call "Dark Side" powers, provided that they are trying to remain good...but that this general attitude can all too easily lead to people doing great evil and claiming they're doing it for the right reasons. (See also, Kylo Ren.) And, ultimately, that'll be the conflict we see between Rey and Kylo.