From the article:
What Is Groupthink?
Janis’s first rule is that a group of people come to share a particular way of looking at the world which may seem hugely important to them but which turns out not to have been based on looking properly at all the evidence. It is therefore just a shared, untested belief.
Anyone holding a contrary view must simply be ignored.
Rule two is that, because they have shut their minds to any evidence which might contradict their belief, they like to insist that it is supported by a “consensus.” The one thing those caught up in groupthink cannot tolerate is that anyone should question it.