In recent years, the usages of public key infrastructure( PKI) in secure e-mail service, e-commerce service, client authentication service with SSL, etc. have increased. However, a PKI faces many challenges in the practice, especially the scalability of the infrastructure. ID-based cryptosystem(ID-C) has been proposed to solve the problems of PKI by eliminating the necessity for the infrastructure to authenticate public keys and manage directories to store certificates. But, the key escrow is integrated in this setting such that Private Key Generator can easily threaten security of Agents. In this paper, to enhance the security of Multi-Agent in distributed computing environment, we first propose an ID-based threshold decryption scheme without key escrow which has a lost share recovery property. Also, the proposed scheme can provide the group division/merge and key update scheme for a dynamic group membership.