It is illegal to coerce you to join a union. This can be a thin line. It is true that a union cannot force you to join. However, you might find that it is clearly in your best interest to join. Actors cannot get roles in major motion pictures unless they are part of the Screen Actor's Guild or they can somehow convince the production company to pay a fine for using a non-union actor. While no one can force you to join a union there may be practical considerations that make it very difficult to not join if you want to work in a particular industry.