The employees are most likely very good and loyal friends of Otto's. They are also likely Jewish and part of a resistance group which is why they are helping Otto and his family hide their business from the Nazi's. That would also explain how they are getting supplies. It is also probably how they were able to get news because when Otto was in hiding he likely wouldnt have any kind of radio and many Jewish resistance groups at the time had Nazi radios so they could get news from the outside world and so they could know what the Nazi's were doing so they wouldnt get caught.