The NLRB is known primarily as the board that governs unions and union organizing. Well, for many years they've considered employees discussing salary info as protected organizing activity. Any employee that experiences adverse action by an employer after discussing salary with a coworker should contact the NLRB. They will pursue the issue on the employees behalf.
This advice comes from my lawyer, but you should consult your own before you do anything. Also, my 0.02 is that as an employee, I feel I'm often at the higher or top end of my employers range, and despite the fact that I legally COULD, I don't really see any value in discussing my pay with a coworker.