Deindividuation is when individuals in a group abandon their normal reservations about certain actions in order to fit in with the group. In other words, doing something in a group that we would not have done alone. This has been an issue throughout the history of society because although individuals may have been moral beings before entering a group, as soon as they entered many groups have done some horrific things (such as the Ku Klux Klan and the Nazis).
In the following clip, Lupin from A Very Potter Sequel illustrates deindividuation when the group decides to make fun of Hermione. Although this is a light view on the power of group pressure, it still provides a valid example. (skip ahead to 0:20)
As you can see, Lupin originally enters the scene trying to protect Hermione by stating that the song is not even that funny. That caused the group to turn on him by changing the song to “Lupin Can’t Sing,” and after a few seconds of trying to convince the group that he could sing, he gave in and refocused the attention on Hermione. Therefore, he abandoned his original beliefs when faced with the group’s pressure and specific rules. Instead of continuing to fight the group, Lupin gave in and experienced deindividuation.