McCormick Co., an independent communications and advertising agency with an office in Des Moines, will join along with two other companies the network of global advertising agency Sandbox, the company recently announced. Chicago-based Sandbox is a full service advertising agency established through the combination of complementary entrepreneurial firms that stress the importance of collaboration. With these additions, sandbox will employ 337 people and have a substantial presence in eight U.S. and Canadian cities, including Des Moines.