A group formation/management system rigidly sets a group range, allows contents to be used freely among member devices in the group, and includes one or more registered member devices for holding common secret information unique to the group, a new member device for transmitting a request for registration in the group and receiving and holding common secret information, and a group management device for receiving the registration request from the new member device and, when the number of registered member devices is less than the maximum number of registerable member devices, outputting the common secret information to the new member device. Furthermore, because member devices are authenticated using the common secret information when contents are to be used, and contents are only delivered if the authentication is successful, member devices that do not hold the common secret information (i.e. unregistered member devices) can be prevented from using contents.