The authentication is one of the important problems in Cloud computing environment. In this paper, a new Group Key Authentication (GKA) protocol is proposed. This protocol reduces the authentication time and data traffic in the Cloud computing and simultaneously increases the Quality of Service (QoS). Two additional aspects need to be addressed in cloud computing environment are load balancing and robustness. To solve these problems, mathematic grouping and heuristic grouping strategies are also proposed in this paper. In addition, it also describes a new access control policy based on group signatures in cloud service. To build this access control policy scheme, a double center access control model is designed. This scheme is able to manage multi-groups and validate the group certificates of users. Extensive evaluations have been conducted on the analytical model to verify the effectiveness and the correctness of our strategies.