This Chinese symbol represents the harmonizing forces of the universe. It is believed that the outer circle symbolizes "everything", while the black and white shapes within the circle represent the interaction of two energies, called "yin" (black) and "yang" (white), which cause everything to happen. They cannot exist without each other. One is not better than the other but too much of either one is bad. The ideal is a balance of both.
I think this applies to the Joy Luck Club because the Yin-Yang is present in the Chinese culture. I also think that the idea of the black and white balancing each other can relate to many of the woman's lives. And together the Joy Luck Club is balanced with each of the four lady's qualities and traits. They are all so different but together they are balanced.