I work in the advertising industry as a free-lancer for production companies that service ad agencies that produce television commercials. May I put money in the Social Security system?
If you are self-employed operating a trade or business, then the answer is yes, assuming that you have a net profit of at least $400 annually. In fact, it is actually required. For more information, see this pamphlet from Social Security: https://www.ssa.gov/pubs/EN-05-10022.pdf.