When people collectively make a decision or state an opinion, especially one that seems foolish, they're using groupthink. If you go along with your friends' idea to jump off a moving hayride together, you're a victim of groupthink.
The word groupthink is most commonly used in an office or business context. This phenomenon occurs when people who like and trust each other go along with an idea without stopping to think it through critically. It first appeared in Fortune magazine in 1952, inspired by George Orwell's 1984 and its terms like "doublethink." Today groupthink is considered a psychological phenomenon that occurs when conforming to a group feels more important than reason and rationality.