Answer:
Judas shared the sop or piece of bread and then immediately left before the cup was offered.
It is interesting to note that the cup symbolized the blood of Christ and the remissions of sin, which Judas rejected both literally and spiritually by not repenting of his wrongdoings but instead taking his own life.
Jesus himself did not drink the cup that night with his disciples either but for much different reasons. Matthew 26:29 But I say unto you, I will not drink henceforth of this fruit of the vine, until that day when I drink it new with you in my Father’s kingdom.
The cup which came after dinner was symbolic not only of the remission of sins but of the joy that comes from this. Jesus told His disciples he would not partake of that with them until it was in His Father’s kingdom. Jesus knew that while it symbolized remission of sin and joy for His disciples, it meant the Lamb of God would be brutally sacrificed for this to be fulfilled. Jesus did receive this cup when He ascended into Heaven and presented His blood once and for all on the mercy seat in Heaven. Then He too rejoiced in Spirit for the reconciliation with His Father and the reconciliation of all believers (you and I) to His Father as well.
I learned most of this in Israel when I was able to re-enact much of the Passover feast with a wonderful Messianic Jew I met while traveling there. It was a very moving experience, one I will never forget. Think what it must have meant to the disciples as they thought back on that night years later!
Very good question,
Pastor Brad