What's the difference between "agency", "group" and "organization"?

Native speaker’s answer
Rebecca
An "agency" is a business that represents one group of people when dealing with another group. An "organization" is a group of people who work together in an organized way for a shared purpose. "Group" has a couple of different meanings. One meaning is several people or things that are put together or considered as a unit. Another meaning that is closer to the other definitions is a business that contains several different companies. Ex: She works for an advertising agency. Ex: SMILES is a non-profit organization that supports adults with disabilities. Ex: The group owns a bunch of different companies overseas.