Which individuals would belong to the Roman Empire's common class?

Answer 1

A very wide range of people. There were two classes of society in Rome: Patricians and Plebeians. Patricians originally only were the oldest established, ruling families and clans of Rome who had representatives in the Senate. Plebeians was everyone else.

Over time and certainly during the age of the Emperors, many Plebeians became rich and powerful and the "upper" Plebeian group called the Equestrians (originally Plebeians who had done army service as cavalrists) finally came to hold most executive jobs in the Empire.