Z.F.
C.
What is the description of the population you are looking for? Are they college students? are they parents? Human Resources personnel?
Once you establish a description you can pin-point where to advertise.
Doing some web-research (e.g. who is the personnel director at a particular company?) and sending emails might work.
Send flyers to companies in the area where you are offering the workshop. I would include a description of the program (as well as the title) some background of the instructor, and how much it will cost. You might want to give a discount for more than 3 or 4 attendees.
Good luck - sounds like an interesting workshop