There were many ethnic groups in pre-Roman Italy. The most notable, and the ones who help tremendous power, were the Etruscans. The Etruscans held lands north of Rome on the Western coast (see the first linked image). They were known for their unique art style, pottery, and sarcophagi. The Etruscans functioned as a kingdom, in which the Romans were among the lower classes. Under the final Etruscan king, Tarquin the Proud, the Romans staged a coup, kicked the Etruscans out of power, and declared the land as a Roman Republic in 509 BCE. The Romans then separated their social system into Patricians, or the upper class, and Plebeians, or the lower classes. The Etruscan civilization blended with the Romans until it eventually died out/merged with Rome.
Another ethnic group in pre-Roman Italy were the Greeks. The Greeks were in the midst of their golden age when Rome was forming in the late sixth century BCE to early 5th century BCE. They occupied territory south of Rome and on the modern-day island of Sicily (see the first linked image). The Greeks, like the Romans, were seagoing peoples who expanded their territories and spread their culture and language throughout the Mediterranean world. It is no secret that Rome's pantheon of gods was taken from the Greek's, only renamed.
A third major ethnic group were the Umbrians. They had territory east and northeast of Rome (see the second linked image).