An old Candid Camera video, using the Asch experiment, shows the power of "group-think" but Brain Pickings reports that having just one colleague express a different opinion made others eager to share their opinions.
Having just one peer contravene the group made subjects eager to express their true thoughts. Surowiecki concludes:
Ultimately, diversity contributes not just by adding different perspectives to the group but also by making it easier for individuals to say what they really think. [...] Independence of opinion is both a crucial ingredient in collectively wise decisions and one of the hardest things to keep intact. Because diversity helps preserve that independence, it’s hard to have a collectively wise group without it.”