In conference channels, users communicate with each other using a conference key that is used to encrypt messages. There are two basic approaches in which the key can be established. In the first one, a central server is used (with a chairman role). The server generates the key and distributes it to participants. The second approach is that all participants compute a key without a chairman. In this paper, we introduce a special type of group authentication using secret sharing, which provides an efficient way to authenticate multiple users belonging to the same group without the chairman. Our proposed protocol is a many-to-many type of authentication. Unlike most user authentication protocols that authenticate a single user each time, our proposed protocol authenticates all users of a group at once.