The board of trustees shall elect from its membership a chairman, and shall by majority vote of its members appoint a secretary who may be one of its members. It may engage such actuarial and other services as may be required to transact the business of the retirement system. The compensation of all persons engaged by the board of trustees and all other expenses of the board necessary for the operation of the retirement system shall be paid at such rates and in such amounts as the board of trustees shall approve.