Any unincorporated association that is an organized medical society limiting its membership to licensed physicians and surgeons and that has as members at least 25 percent of the eligible physicians and surgeons residing in the area in which it functions (which must be at least one county) may, without incorporation, purchase, receive, own, hold, lease, mortgage, pledge, or encumber by deed of trust or otherwise, manage and sell all the real estate and other property as may be convenient for the purposes and objects of the association. However, if the association has less than 100 members, it shall have as members at least a majority of the eligible persons or licensees in the geographic area served by the particular association. The members of that unincorporated association are not individually or personally liable for debts or liabilities contracted or incurred by the association in the acquisition of lands or leases or the purchase, leasing, construction, repairing or furnishing of buildings or other structures to be used for the purposes of the association or for debts or liabilities contracted or incurred by the association in the carrying out or performance of any of its purposes; provided, that the purposes are within the purposes stated in Section 18020.