When people are part of a group, they have a tendency to experience deindividuation - that is, a loosened sense of self-awareness - and are less inclined to feel a responsibility for their actions (which causes a loss of inhibition and restraint).
Which is to say, you’re more likely to feel a certain way or do a certain thing you ordinarily wouldn’t if everyone else is doing it.
The larger a group is and the more anonymous individuals feel within that group, the easier it is to lead that group to a common feeling or to cause it to engage in a common action. It only takes a small push to provoke the reaction you’re looking for, though this particular social influence is usually correlated with violence. Take advantage of the situations you’re in.