In network communication, security is essential to prevent intruder's attack; Where Authentication plays a vital role so as to ensure the right participant in communication. To authenticate a group of users and to hold confidentiality between them a new approach of key management is handled in this paper. This key management presentation is a group key used for cryptography process that provides security with less computation complexity, which leads to fast and easy authentication. Here, a tree based key management scheme has been designed through which the key distribution among the users involves authentication with less storage complexity too. This paper describes the dynamic membership authentication, which states the communication or transaction between user and required server with a new different instant key that is formed from a group of members who are currently participating in transaction or communication. Releasing or joining a user from the group or to the group is possible so that the key gets changed instantly whenever there is an alteration in current participation of users in communication. With the help of each user's hidden key an 'Instant Key' can be computed. As the entire user's hidden keys are used for obtaining an Instant key, it can ensure the authentication among the valid users during communication. So an intruder cannot interrupt any communication between users as the key gets varied often. Thus it supports clear authentication and provides security among group communication. ©2006 IEEE.