You may want to wait until the publication of the Gospel of Judas. That might be specific on Judas' presence at the Last Supper.
The extant gospel material is pretty sketchy. In Mark 14:17-18, Jesus was seated "with the twelve" as they ate, presumably the "passover" of v. 16, and Matthew similarly has Jesus "at the table with the twelve disciples" (Matthew 26:20-21). In his prediction of his betrayal, Jesus specifically mentions Judas' presence at the table (Mark 14:18; Matthew 26:21), and no mention is made in these accounts of Judas' departure from the room. Rather, in the account of the institution of the Eucharist the pronoun autón "of them" in Mark 14:22 and Matthew 25:26 has the "twelve" as its antecedent, so the most felicitous interpretation (tho by no means secure) is that Judas was still there. The text however just doesn't say this explicitly. The account in Luke is somewhat different. In Luke 22:14, Jesus is simply accompanied by the "apostles"; they are not numbered and Jesus' prediction of his betrayal follows the institution of the Eucharist, rather than preceding it as in Mark or Matthew. Thus, according to Luke, Judas was definitely present there during the Eucharist. But none of the synoptic gospels mention his departure from the room. Rather, his departure was noted in John (cf John 13:30), but the meal here is explicitly not the passover and the text does not describe any institution of a Eucharist. Thus, John 13 does not clarify the matter; it is belongs to an independent tradition. The text includes a prediction by Jesus of his betrayal (13:21-26), but at the time of the prediction the disciples were dipping morsels into bowls -- which sounds like the kind of meal that might precede the Eucharist, and it is possible that the interpretation that Judas left before the Eucharist arises from harmonizing John with the synoptics, which would place the event described in 13:21-30 immediately prior to the synoptic narrative of the institution of the Eucharist and assume that the author of John simply omitted this later event. Such a harmonizing interpretation however flatly conflicts with Luke.
As for Judas' presence at the meal, here are some old Watchtower articles that claim that he was not present during the institution of the Eucharist:
*** w56 6/15 p. 383 Questions from Readers ***
?
Speaking of those who partake of the Lord?s evening meal unworthily, the January 15, 1956, Watchtower said on page 60, paragraph 18: "So let him benefit by the corrective, disciplinary judgment that Jehovah gives him. Let him discern what he himself is, and reform. If he has been at fault, yet he should obey the command and eat the Lord?s evening meal, but do so discerning the Lord?s sacrificed body and asking forgiveness for his sin. Then let this celebration strengthen him to follow Christ?s steps more closely during the coming year." Does this mean a disfellowshiped person should partake of the loaf and wine at the Lord?s evening meal??M. P., Germany.This has no reference to persons that are in a disfellowshiped condition, but to only those who are continuing in the body of Christ but who are yet subject to sin and who therefore commit sins for which the sacrifice of Christ provides the basis for forgiveness. A disfellowshiped person is no member of Jehovah?s congregation and may not be served the emblems by those serving at the celebration of the Lord?s evening meal. Jesus did not serve Judas the emblems but sent him out of the house before instituting the Lord?s evening meal.?John 13:21-31; Matt. 26:20-25; Mark 14:10-21.
[Note that John is cited in harmonization of Matthew and Luke, whereas Luke -- which presents the opposite view -- is conveniently omitted]
*** w57 1/1 p. 31 Questions from Readers ***
?
Was Judas present when Christ instituted the memorial celebration or not??W. E., United States.By comparing Matthew 26:20-25 with John 13:21-30 we learn that Judas was not present when Christ instituted the Lord?s evening meal. One Gospel account tells some details that the other one does not tell, but by harmonizing the two the complete picture can be obtained. Matthew tells us how a questioning arose as to who would betray Jesus. John tells us that before that subject was dropped the betrayer was identified by being given the morsel that Jesus had dipped, and immediately Judas went out into the night. Matthew goes on to describe how Jesus then served the remaining eleven apostles the emblems of the memorial, while John, who wrote his account after Matthew did his, does not repeat the procedure of the evening meal but instead gives some of the extended remarks Jesus made on that occasion, as recorded in chapter 13. Jesus? discoursing and praying with his disciples, however, as recorded at John chapters 14 to 17, were not part of the Lord?s evening meal, but came after it.
Thus do these two apostles and eyewitnesses of the described events agree as to the time element, for John?s account in no way contradicts Matthew?s as to the time of departure of Judas. The account of Luke (22:14-23) is not to be taken as contradicting this time order either. Luke was not an eyewitness of the events. He tells of the same events, but he does not necessarily do so in the exact chronological order, as do the two who actually witnessed the occasion. Additionally, Luke 22:28-30 could not have included Judas, so he must have left prior to its being stated. See The Watchtower, January 15, 1951, pages 46 and 61.
[Here again the claim is made on the basis of harmonizing John with Matthew. Luke is arbitrarily dismissed as not necessarily chronological, without any reason given for why this is the case -- other than the tradition that John and Matthew were written by eyewitnesses, which doesn't stand up to critical scrutiny. Luke 22:28-30 is also no clear support that only eleven apostles were present, as it mentions the apostles on "thrones judging the twelve tribes of Israel" and the parallel in Matthew 19:28 mentions "twelve thrones judging the twelve tribes of Israel".]
