The petitioners shall designate in any initiative, referendum, or supplementary petition and on each of the several parts of such petition a committee of not less than three nor more than five of their number who shall represent them in all matters relating to such petitions. Notice of all matters or proceedings pertaining to such petitions may be served on said committee, or any of them, either personally or by registered mail, or by leaving such notice at the usual place of residence of each of them.