|John on the Isle of Patmos|
There is a bank robbery. Four tellers are interviewed by investigators. At the trial, the investigators state, “The four witnesses agree that the bandit wore a red shirt and black pants.” The defense attorney asks the investigators if they interviewed the tellers independently or as a group. I think you'd agree that independent corroboration should be considered more potent and that a group interview brings in questions about groupthink errors, such as, one teller remembered the clothing and the other three agree with him. Don't biblical scholars agree that the Gospels show signs of having some common source? I believe it is known as Q. And given the time difference between the events and the documentation, should we at least suspect that a number of the “important details” had already been accepted?
The three synoptic Gospels--Mark, Luke, and Matthew--include the same stories, often in the same sequence, but the Gospel of John is different. Scholars believe that Mark is the earliest Gospel, followed by Matthew and Luke, and then John.
The reason why scholars have hypothesized a Q source is because Matthew and Luke contain material that doesn't exist in Mark, and the idea is that Q is a collection of sayings and quotations by Jesus. However, they are not entirely happy with that hypothesis because it seems unlikely that such an important document would have been entirely lost and never referenced.
But for the purpose of answering your question about groupthink, I'm going to focus on Luke and John. Luke wrote both the Gospel of Luke and the Acts of the Apostles to "the excellent Theophilus." Luke 1:1-4 says: "Inasmuch as many have undertaken to compile an account of the things accomplished among us, just as they were handed down to us by those who from the beginning were eyewitnesses and servants of the word, it seemed fitting for me as well, having investigated everything carefully from the beginning, to write it out for you in consecutive order, most excellent Theophilus; so that you may know the exact truth about the things you have been taught."
So we know that he is writing to someone who is important enough to need his excellence affirmed in the salutation, and that he is purporting to give a detailed, consecutive account of the events of the life of Jesus. In other words, Luke probably held himself to a high standard in researching and writing the Gospel of Luke and the Acts of the Apostles.
The question then becomes whether we have more than Luke's word for his factual accuracy. The world-famous archeologist, Sir William Ramsay, has said, "Luke is a historian of the first rank . . . This author should be placed along with the very greatest of historians." Do we have reason to accept his assessment of Luke? Yes we do, because the historical accuracy of the book of Acts is indisputable. For example, according to Professor A. N. Sherwin-White, author of Roman Society and Roman Law in the New Testament, "For Acts the confirmation of historicity is overwhelming. Any attempt to reject its basic historicity even in matters of detail must now appear absurd."
Also, Luke begins to use the first person plural starting with Acts 16:11, indicating that he joined Paul in an evangelistic tour of Mediterranean cities. This is consistent with his statement at the beginning of the Gospel of Luke that he got his information from eyewitnesses and "servants of the word."
Whereas the Gospel of Luke purports to be a detailed, chronological history of Jesus, the Gospel of John is a deeply spiritual, reflective work with a high Christology, which means that it is more concerned with the nature of Jesus than with the details of His life. Rather than starting with the beginning of the life of Jesus, it starts with the beginning of time, and talks in a simple, mystical way of how the logos--the eternal transcendent Mind--became flesh and dwelt among us.
John is an entirely independent source, and scholars are split in terms of whether they think it was written by the apostle John himself or a follower. Although I see no reason to doubt the authorship of John (the work was attributed to John as early as the second century), even if one of his disciples wrote the Gospel the material would have originated from John himself, who was an eyewitness and very close to Jesus, calling himself "the disciple whom Jesus loved." In no way is he claiming that Jesus loved him more than the other disciples; he is basing his identity on his deep, personal awareness of God's love for him. In his epistles he emphasizes love as the essence of God's nature, so it is fitting that John's Gospel would focus on Christology.
So although the two Gospels cover the same events, Luke and John are very different in terms of their focus and style. Most likely they had little, if any, influence on each other.
Many people knew about the events (in Acts 26:26, Paul says that he's confident that King Agrippa knew about the events, because they did not take place in a corner), so it would have been very difficult for groupthink about important details to emerge so soon afterwards. And the fact that even the synoptic Gospels differ in the minor details indicates that there was no collusion. Even if Luke and Matthew based their account in part on Mark, they included details that were not found in Mark.