Josh Groban, who performs at the Capital One Arena tonight, is being urged to end unfair hiring practices for his back-up musicians. “Back-up musicians who have been contracted to play for your tours have reported low pay and difficult working conditions,” the DC Musicians Union says in a full-page ad in Thursday’s Washington Post Express. “Area sub-contractors working on your behalf are engaged in unethical hiring standards,” the ad says. The union says orchestra musicians were denied access to an air conditioned tent in sweltering heat, received no food or per diem, no benefits and no travel mileage. While noting that “We have no reason to believe that you know about these unfortunate practices,” the musicians’ union urges Groban to act, saying that “the Example You Set Matters.”