According to John, what responsibility did Judas Iscariot have among the disciples?

Answer: He kept the money bag

Judas was keeper of the money bag, and John says he was a thief who used to take from what was put into it.

“Judas did not say this because he cared about the poor, but because he was a thief. As keeper of the money bag, he used to take from what was put into it. Since Judas kept the money bag, some thought that Jesus was telling him to buy what was needed for the feast, or to give something to the poor.”— John 12:6, 13:29 (BSB)