Recently, there has been extensive research on integrating cellular networks and ad hoc networks to overcome the limitations of cellular networks. Although several schemes have been proposed to use such hybrid networks to improve the performance of individual multicast groups, they do not address the Quality of Service (QoS) issues when multiple groups are present in the networks. Our work, on the other hand, considers an interesting scenario of hybrid networks when an ad hoc network cannot commodate all the groups and a base station has to select a subset of groups to optimize its bandwidth savings and maximize the utilization of the ad hoc network, while providing QoS support for multicast users. In this work, we develop a network model for multicast admission control which takes wireless interference into account, formulate the group selection problem as a multidimensional knapsack problem, and propose an Integer Linear Programming (ILP) formulation and a polynomial-time dynamic algorithm. We also examine a distributed implementation of the dynamic algorithm in real systems. Simulation studies demonstrate that the dynamic algorithm is able to achieve very competitive performance under various conditions, in comparison with the optimal solution computed by the ILP approach.