Communications for mobile sink groups such as rescue teams or platoons bring about a new challenging issue for handling mobility in wireless sensor networks. Recently, a group region-based approach has been proposed to send data to a mobile sink group. However, it uses flooding for discovering and notifying group region information and flooding or long distance forwarding for disseminating data. Thus, it causes high energy consumption and data delivery failure. Moreover, its per-sink mobility supporting scheme makes energy consumption and data delivery failure more serious. Thus, this letter proposes an Energy-efficient and Reliable Mobile Group communication protocol (ERMG) to support a mobile sink group, which exploits a virtual grid structure. For the group region discover and notification and the data dissemination, ERMG uses communications with only cell heads of grid cells that manages group sinks. ERMG also uses a per-grid based scheme to support sink mobility. Simulation results show that ERMG has better performance than the existing protocols.