The branch of science that studies groups of cells and how they work together is histology.

Here's a bit more detail:

* Histology is the study of the microscopic structure of tissues. Tissues are groups of similar cells that work together to perform a specific function. Histology examines the arrangement, structure, and function of these cells and the extracellular matrix that surrounds them.

* Cell biology, while closely related, focuses on individual cells and their components.

* Physiology explores the functions of organs and organ systems, which are made up of tissues.

So, while cell biology and physiology are important for understanding how groups of cells work together, histology is the specific field that focuses on the structure and organization of these groups.