§ 90. Dividends. Policyholders insured in the state insurance fund may be divided into such groups as shall be equitable for the purpose of accounting and declaration of dividends but for the purpose of paying compensation the state fund shall be deemed one and indivisible. Separate accounts shall be kept of income and of losses and expenses incurred, including contributions to catastrophe surplus and reserves adequate to meet anticipated losses and carry all claims to maturity, for each such group. If such accounting shows a balance remaining to the credit of the group at the close of any policy period, which shall be deemed to be safely and properly so applied, there may be credited or paid to each individual member of such group such proportion of such balance as the amount of his earned premium sustains to the total earned premium of the group for the period for which the accounting is made. If any member who has withdrawn from the group would otherwise have been entitled to such a dividend, the same may be credited or paid to him.