This paper presents a dynamic group mobility (DGM) model for hybrid networks. In this model, the ad hoc network is placed in the cellular system. In ad hoc network, mobile nodes (MNs) can dynamically re-configure themselves triggering group partition and mergence. Therefore, MNs sometimes merge and move together and sometimes split and move individually. In cellular system, when group MNs roam from one local area to another, the group leader ensures the location update for the whole group. Since it is too expensive for each MN within the group to communicate all the time with the base station (BS), we designate the group leader to ensure the location update and the administrative tasks for the whole group. This allows to reduce the power consumption, and to limit the wireless bandwidth used.