This article presents a method for evaluating the distribution of the external loads within a pile group. The model is based on the imposition of the rotation compatibility condition between the pile cap and the piles heads. It includes the soil-foundation interaction which depends, in turn, on two main parameters. The result is a set of clear formulas similar to those provided by the statically-determinate method. Finally, a parametric analysis shows the correlation, by means of an interaction domain, between the axial forces and the bending moments in the pile heads as a function of the lateral loads.