A group key agreement protocol is a process to establish a cryptographic key for a group of participants over an open network. In this paper, we propose a group key agreement (CAGKA) protocol, based on a certificateless public key cryptosystem. CAGKA protocol provides group key establishment and group membership change (join and leave) services for dynamic groups. This protocol is proved to be secure against passive and active adversaries and is more efficient than previous group key agreement protocols.