Sociology is the study of human development within a society. To that end, it is important to understand that groups contribute many of the ideas that sociologists analyze. For example, a society is made up of various cultures within a defined populous. Culture is pervasive and widespread throughout all of sociology. But on its most basic level, culture is defined by groups of people creating living norms and standards that are acceptable by the group as representative of their status quo.
More plainly stated, sociologists do not study individual behaviors inside of a vacuum. Instead, sociologists study human interactions, processes and cultural parameters to understand the society as a whole. They can then formulate ideas about the human condition based on how groups feed off of each other,and everyone outside of their group. So, without groups, there would be no fundamental understanding of a society at all, because there would be no understanding of what is considered acceptable, unacceptable, successful, outlier, or other. Groups also help individuals to form an identity. Again, the group creates an understanding of the world from which to base development.
In sociology groups are studied because humans interact in groups. Human beings are beings who each need the interaction with other human beings.
Human beings in relation to business and groups have a tendency to look at things in a cultural manner and have established mores which also help to influence the items which humans purchase. For example, a flat screen television with high definition is a purchase which many people in American or Western world homes identify as a want or must have. In the outback of Australia where indigenous people still live the way their ancestors lived, such a purchase would seem crazy.
People respond to one another and learn their values and morals from the group they inhabit. If a person feels like an outsider in a group, he will seek out those who are like him so that he can find a place where he feels that he belongs.