Answer:
The symptom that led to groupthink was the general idea that the arguments heard in court were convincing enough to declare that the defendant was guilty.
The person who stook against it was jury #8, played by Henry Fonda, who felt that it was unfair to declare that the defendant was guilty without giving a harder look to the situation first.
In the end, jury #8 managed to sway the other 11 juries because he was able to spot several holes in the arguments of that they heard in the court, and he communicated those holes in a very convincing manner.