In The Last Jedi, it is mentioned that some students followed Kylo Ren on the run from the academy. In a document, are you informed of the fate of these students?
The Knights of Ren were not Force-Sensitive (at least that is how they are portrayed) so it is unlikely (but plausible and possible) that these other students joined the Knights of Ren. Another answer that has come to my attention is that these students that left with Ren could have became the Red guards seen in Snokes throne room, as suggested by Star Wars Explained -
EDIT: Some members of The Knights Ren probably were force sensitive (with the exception of Kylo Ren), but this has not been shown yet in canon (as of December 2017.)