Group communication implies a many-to-many communication and it goes beyond both one-to-one communication (i.e., unicast) and one-to-many communication (i.e., multicast). Unlike most user authentication protocols that authenticate a single user each time, we propose a new type of authentication, called group authentication, that authenticates all users in a group at once. The group authentication protocol is specially designed to support group communications. There is a group manager who is responsible to manage the group communication. During registration, each user of a group obtains an unique token from the group manager. Users present their tokens to determine whether they all belong to the same group or not. The group authentication protocol allows users to reuse their tokens without compromising the security of tokens. In addition, the group authentication can protect the identity of each user.