In the theory of collective behavior, it is important to separate the organized action of a group from the diffuse, aggregated actions of individuals. Both are of interest to social psychologists, since they have to do with a mass of people. However, groups have common interactions and relationships within it, while the interactions and relationships within an aggregate are diffused.
For the purposes of social psychology at the wider level, there are two fundamental worries. First, what acts as the catalyst for emergence of groups and norms, and what maintains those social forms? Second, how do social perceptions occur, especially with respect to primary socialization?