Mobile devices are typically equipped with multiple access network interfaces, supporting the coexistence of heterogeneous wireless access networks. The selection of an optimal set of serving mobile networks for multicast streams is a challenging problem. We consider a network selection problem for multicast groups of mobile clients that operate in a heterogeneous wireless access network environment. We present a solution to this problem with an optimal allocation of mobile users to multicast groups when multiple mobile networks are available for operation. This solution is suited for small scale networks and can be used as reference for complex networks.