The text does not say this one was "IN LEAGUE WITH" the high priest. It says he was "KNOWN UNTO" him.
There was another "disciple" that was in these general scenes... the one in Mark 14:52, the one who "fled naked" but only after (and because) they "laid hold on him" (i.e. onto his "linen cloth"). I do not believe this was "MARK" as many suggest. I believe it was "Lazarus," and Lazarus was now FAMOUS, you know... because after Jesus raised him from the dead, MANY JEWS now started to believe on Jesus, and the high priest (and the others in league with him, i.e. "the chief priests") wanted to KILL HIM TOO, per John 12:9-11 (which they weren't going to do, at THIS scene, Jesus' arrest and trials).
[I don't believe it was "JOHN," either, because later, much later in Acts 4:13, it says of John that "When they saw the boldness of Peter and John and realized that they were unschooled, ordinary men, they marveled and took note that these men had been with Jesus." Saying "took note that these men had been" is not suggesting WELL-KNOWN unto them]