Russel Williamson has joined Watertown-based Century 21 Commonwealth Commercial as an advisor. Williamson will work for the branch office at 161 Mount Auburn St. "Commonwealth Commercial has helped me to hone my industry knowledge of the commercial real estate market and provide referrals to those customers who wish to make a commercial property investment, whether they would like to invest in a office building in Brookline or in an apartment complex in Boston," Williamson said.