Many users who are in a cyber-space usually want to join the social group to have or to share their information. Now, there are two ways to join the group, the group manager invites them, and the users who want to join ask the owner. These days the group polices usually follow this way. But, it can be faced a security problem when the manager send group messages in near future because they don't have any securities. Therefore, the security modules to join groups will be needed when they join the group or when they read the group messages. To set the security, we have to think how to keep the key such as a generation /an update/an arrangement, because all users need the key to join the groups or to read the group messages by decrypting. The key are going to be used to joining the group when it dynamically changes such as frequent group joining and leaving. If it applies or uses the existing methods in the smart cities which consider the users who will move globally, it could easily assume that the overhead/the cost of CPU will be increased and it follows capacity down because of lots of the key updates. So, to let them down, we suggest three key strategies, a group key, a subgroup key and a session key in this paper.