Section 141 of the N.I Act, states that if the person committing an offence under Section 138 is a Company, every director of such Company who was in charge of and responsible to that Company for conduct of its business shall also be deemed to be guilty. The reason for creating vicarious liability is plainly that a juristic entity i.e. a Company would be run by living persons who are in charge of its affairs and who guide the actions of that Company and if such juristic entity is guilty, those who were so responsible for its affairs and who guided actions of such juristic entity must be held responsible and ought to be proceeded against.
If any of the directors' contend that the offence was committed without their knowledge or that they had exercised due diligence to prevent such commission, it would be a matter of defence to be considered at the appropriate stage in the trial.
Therefore you may file a petition to implead other directors in the present private complaint, let the court decide about it after hearing both the sides.