Marketing is not a department, it’s your whole business
Everyone in the company, from the cleaner to the man at the top, can be communicating positive messages about the company whilst at work and outside of work. What you need to do is encourage them to do so, and to give them good reason to do so. Think about it, no one is going to say good things about your company if they are unhappy working there, or if they know the company’s products are rubbish. Help them to be proud of what their company does and encourage them to communicate their pride.