Contact you state department of labor (or if you don't get helpful responses from them, the federal department of labor). All employees must be paid at least minimum wage (and overtime when they work over 40 hours per week, unless except from overtime, which these workers would not be) under the labor laws (e.g. Fair Labor Standards Act or FLSA) and the departments of labor enforce these laws. The should investigate this matter and help the workers get the back wages to which they are entitled. The dept. you contact will let you know what information, evidence, or forms they need.