Whenever it is charged by any interested party that any person has engaged in or is engaging in any such unfair practice, the board, or any member or designated agent thereof, shall have authority to issue and cause to be served upon such person a complaint, stating the charges in that respect, and containing a notice of hearing before the board, or any member or designated agent thereof, at a place therein fixed, not less than five days after the serving of said complaint. Any such complaint may be amended by the board, member or agent conducting the hearing at any time prior to the issuance of an order based thereon. The person so complained of shall have the right to file an answer to the original or amended complaint and to appear in person, or otherwise, to give testimony at the place and time set in the complaint. In the discretion of a member or agent conducting the hearing or of the board, any other person may be allowed to intervene in the said proceeding and to present testimony. In any such proceeding, the rules of evidence prevailing in courts of law or equity shall be followed but shall not be controlling.