Ad agency holding company, The Interpublic Group, has acquired a minority stake in Endeavor Marketing, the corporate entertainment marketing division of talent agency Endeavor, and has entered into a consultancy agreement with Mark Dowley, the division’s chief executive. Endeavor Marketing clients include American Express, but the firm works mostly on a one-off project basis with advertisers. Mr. Dowley had worked with Interpublic previously and will be retained by the company on an exclusive basis.