At any time after the institution of proceedings and before the hearing examiner makes his decision, the hearing examiner may, upon petition in writing and for good cause shown, permit any interested person, including an employer, employee, labor or trade organization, or Federal or State agency, to intervene therein. The petition shall state with precision and particularity: (a) The petitioner's relationship to the matters involved in the proceedings, (b) the nature of any material he intends to present in evidence, (c) the nature of any argument he intends to make, and (d) any other reason that he should be allowed to intervene.