No, a group of similar cells performing the same function is called a tissue, not an organ.

Here's the breakdown:

* Cells: The basic building blocks of life.

* Tissue: A group of similar cells that work together to perform a specific function. For example, muscle tissue, nervous tissue, connective tissue, and epithelial tissue.

* Organ: A structure made up of different tissues that work together to perform a complex function. For example, the heart, lungs, stomach, and brain.

* Organ System: A group of organs that work together to perform a major bodily function. For example, the digestive system, circulatory system, nervous system.

So, while tissues are the building blocks of organs, they are not organs themselves.