Securing group communications in dynamic and large-scale groups is more complex than securing one-to-one communications due to the inherent scalability issue of group key management. In particular, cost for key establishment and key renewing is usually relevant to the group size and subsequently becomes a performance bottleneck in achieving scalability. To address this problem, this paper proposes a new approach that features decoupling of group size and computation cost for group key management. By using a hierarchical key distribution architecture and load sharing, the load of key management can be shared by a cluster of third parties without revealing group messages to them. The proposed scheme provides better scalability because the cost for key management of each component is independent of the group size. Specifically, our scheme incurs constant computation and communication overheads for key renewing. In this paper, we present the detailed design of the proposed scheme and performance comparisons with other schemes. Briefly, our scheme provides better scalability than existing group key distribution approaches.