Group communications is important in supporting multimedia applications. Multicast is an efficient method in implementing the group communications. However, it is challenging to implement efficient and scalable multicast in Mobile Ad hoc Networks (MANET) due to the difficulty in group membership management and multicast packet forwarding over the dynamic topology. We propose a novel Efficient Geographic Multicast Protocol (EGMP). EGMP uses a zone-based structure to implement scalable and efficient group membership management. And a network-range zone-based bi-directional tree is constructed to achieve a more efficient multicast delivery. The position information is used to guide the zone structure building, multicast tree construction and multicast packet forwarding, which efficiently reduces the overhead for route searching and tree structure maintenance. EGMP does not depend on any specific geographic unicast routing protocol. Several methods are assumed to further make the protocol efficient, for example, introducing the concept of zone depth for building an optimal tree structure and combining the location search of group members with the hierarchical group membership management. Finally, we design a scheme to handle empty zone problem faced by most routing protocols using a zone structure.