A professional association may render professional service only through officers, employees, and agents who are themselves duly licensed or otherwise legally authorized to render professional service within this state. The term "employee" as used in this Code section does not include clerks, bookkeepers, technicians, nurses, or other individuals who are not usually and ordinarily considered by custom and practice to be rendering professional services for which a license or other legal authorization is required in connection with the profession practiced by a particular professional association; nor does the term "employee" include any other person who performs all his employment under the direct supervision and control of an officer, agent, or employee who is himself rendering professional service to the public on behalf of the professional association; but no person shall, under the guise of employment, practice a profession unless duly licensed to practice that profession under the laws of this state.