All summer long, from San Diego north to Santa Barbara, pretty secretaries, schoolteachers, stewardesses, housewives and teeny boppers flock to the beaches to play volleyball. In their bikini uniforms they are as much a part of California seashore life as surfers and bronzed Adonis lifeguards. But they are not merely decorations. For the big two-day tournaments, like this one on a recent weekend at Manhattan Beach, the six-girl teams play to win, and they play well. Spectators lounging in beach chairs (right) watched lovely, graceful athletes like Diane Williams (serving at left), and more than once they saw real power volleyball, as when Olympic alternate Beverly Miller (below) leaped and spiked. Enthusiastic girls on such winning teams as the Tyros kicked up sand and gave hip-hip-hoorays for losers, who washed away their frustrations with dips in the Pacific.