An organism that can only exist as a group of cells is called a colonial organism.

Here's why:

* Multicellularity: These organisms are multicellular, meaning they are made up of more than one cell.

* Cooperation: The individual cells within the colony work together, often specializing in different functions to survive. This cooperation is essential for their existence.

* Interdependence: The individual cells are not capable of surviving independently, they rely on the colony for survival.

Examples of colonial organisms:

* Volvox: A type of green algae that forms spherical colonies of thousands of cells.

* Corals: Consist of thousands of individual polyps that live together in a colony.

* Some bacteria: Certain bacterial species form colonies for protection and resource sharing.

Let me know if you have more questions about colonial organisms!