Many group-oriented applications in distributed systems, such as teleconferencing and cooperative works, involve dynamic peer groups. In order to secure communications in dynamic peer groups, authenticated key agreement protocols are required. In this paper, we propose a new authenticated key agreement protocol, composed of a basic protocol and a dynamic protocol, for dynamic peer groups. With the basic protocol, a secret group key can be achieved in a peer group via group handshake, secret broadcast, key derivation and key confirmation phases. By the dynamic protocol, a new secret group key can be reached when member or mass join, group mergence, group division, member or mass quit occurs in a dynamic peer group. Security analysis shows that our protocol offers explicit group key authentication and prevents from both passive and active attacks. In our basic protocol, each group member equally contributes to the secret group key in parallel and guarantees key freshness.