When must an employee in a medical office be trained and in what manner?

This question is too broad to answer. Do you mean an RN an LPN, a receptionist, a bookkeeper, a technician, etc.? Various jobs require different training depending on the duties involved. The individual common thread among workers in a medical office is the entail for privacy, compassion and politeness, which many medical offices lack.
It depends on what they are doing. If it is blood drawing-yes. Shots-yes. Getting a medical history from the tolerant for the Dr-no. Assisting with procedures in the office-up to Dr. It depends on what you are referring to. Some Dr offices do hand on training with the assisting. But for blood work, shots etc...you have to be trained--ie school.