Have you ever been in a group and acted in a manner that was completely out of character for you? How about when you hear on the news that some group of people did something so violent or stupid that you just couldn't believe it? One reason this happens is that people in groups tend to lose some of their own self-awareness and self-restraint when in groups. They become less of an individual and more anonymous. In a sense, people will do things in groups they otherwise would not because they feel less responsible for their actions and less like an individual.
This process of deindividuation can have powerful effects. For example, how can soldiers kill innocent children? They often answer this question by saying that they are not monsters, but that they were going along with the group and that they were just following orders, and that they were not the only ones doing it....all engaged in heinous acts of violence because, in part, they had become deindividuated.
See also: Individuation