Steven Poole:

To call your opponent a victim of groupthink, then, is to ascribe their views solely to their upbringing, area of residence or social associations, and to deny that they are capable of coming to reasoned conclusions on their own. It should hardly need pointing out that consensus on robust scientific theories such as evolution is not groupthink, or that thinking in groups – the ancient philosophical academies, or the 18th-century Republic of Letters, or the modern global academic network – is what has enabled most of the advances of human civilisation. But the modern user of “groupthink” ignores such truths, the better to paint his opponents as intellectual zombies. As used today, the word is therefore a classic example of Unspeak: a rhetorical intervention designed to shut down argument before it starts.

What is really going on when someone complains of “groupthink” is a kind of bovine attempt at self-glamorisation. You follow the herd and parrot groupthink, whereas I am a superior maverick able to think for myself and unmask the nonsense that everyone else believes. This implicit claim, however, is quite severely undermined by the cliche of using the term “groupthink” itself. After all, given that it’s so lamentably common, to accuse others of groupthink is about the most groupthinky thing you can do.

See the passages in How to Think about what C. S. Lewis calls “The Inner Ring.”