Nowadays, group communications are getting more and more popular. In order to secure the communication, all participating users need to share a common group key in advance. The paper proposes a secure and efficient group key distribution protocol based on Shamir’s secret sharing scheme. In the protocol, (1) each user only needs to send registration message in privacy, while all the other messages can be transported in public. Meanwhile, (2) the scheme supports authentication for group keys without any assumption of hard mathematics problem. Moreover, (3) the protocol introduces the notion of on-line/off-line into group key distribution and thus the speeds of group key response and recovery are greatly improved. Analyses show that our scheme is resistant to passive attack, impersonation attack and reply attack.