According to several authors Bion's contribution has been a landmark in the thought and conceptualization of the unconscious functioning of human beings in groups. We provide a mathematical model of group behavior in which heterogeneous members may behave as if shared to different degrees what in Bion's theory is a common basic assumption. Our formalization combines both individual characteristics and group dynamics. By this formalization we analyze the group dynamics as the result of the individual dynamics of the members and prove that, under some conditions, each individual reproduces the group dynamics in a different scale. In particular, we provide an example in which the chaotic behavior of the group is reflected in each member.