BODEGA BAY, CALIF. — In a new service for mutual fund investors, the Benham Capital Management Group allows customers to specify in advance prices to buy and sell within its family of mutual funds, and the order will be executed automatically. The company says the service is an outgrowth of increased volatility in securities markets. Investors could, for example, specify a price lower than the current one and the amount they wish to invest. If the fund falls to that price within 90 days, the shares are bought. If the price does not fall, the order can be renewed.