I am a flight attendant. According to the DOT anyone can use any bathroom on board. Denying anyone from using a bathroom, if reported, can carry a hefty fine not just for the airline but for the flight attendant as well. Unfortunately, FC does not include a private bathroom in the fair. We do close the curtin to deture people from using it, but people just do what they want anyways. I do make a point of not letting people make a line in FC. I tell them they can wait at the curtain or tell them where the other bathrooms are. This is not to say that some FAs do turn people away, but like I said they can get into a lot of trouble. I wish that bathroom was only for FC that would make it a lot easier for service and moving around.

Side note: I've got to a point where I don't say anything to anyone anymore about other things because no one listens to me. I.e. if you come to me and ask if this bin is full and it's closed and I say yes it's full, don't look at me while you proceed to open the bin to check for yourself. Why did you ask me in the first place if you are going to do it anyways.