A group key agreement (GKA) protocol is a mechanism to establish a cryptographic key for a group of participants, based on each one's contribution, over a public network. Security of various group-oriented applications for ad-hoc groups requires a group secret shared between all participants. In ad hoc networks, the movement of the network nodes may quickly change the topology resulting in the increased in e overhead during messaging for topology maintenance, the region based schemes of ad hoc networks therefore aim at handling topology maintenance, managing node movement and reducing overhead. When the group composition changes, group controller can employ supplementary GKA protocols to derive a new key. Thus, they are well-suited to the key establishment needs of dynamic peer-to-peer networks as in ad hoc networks. While many of the proposed GKA protocols are too expensive to be employed by the constrained devices often present in ad hoc networks, others lack a formal security analysis. In this paper, a simple, secure and efficient region based GKA protocol using elliptic curve cryptography well suited to dynamic ad hoc networks is presented. This paper introduces a region-based contributory group key agreement that achieves the performance lower bound by utilizing a novel group elliptic curve Diffie-Hellman (GECDH) protocol and tree-based group elliptic curve Diffle-Hellman (TGECDH) protocol, called GEDH & TGECDH protocol. Both theoretical and simulation studies shows that the proposed scheme achieves much lower communication, computation and memory cost than the existing group Diffie-Hellman and tree based contributory group key agreement schemes.