In this paper, we propose a GKDS that has decentralised group management and is dynamic. That is it allows any user in U, the universe of users, to form a subgroup of existing users and also sponsor a new user to join the group that he initiates. A user v ∉ U that is sponsored by u ∈ U cannot participate in groups initiated by other users. However if enough users in U sponsor v, he will be permanently admitted to U and will have the same capabilities as others. This provides a powerful mechanism for groups to be self-sufficient and grow from a small subset of users without requiring a trusted third party.

We also consider security requirement of join operation in GKDS and show that a recently proposed GKDS [1] although provides secure dynamic controller property but is insecure under its proposed method of user join (using a trusted third party). We give a modification of this system to provide secure user join. We also show that our proposed GKDS provides secure and flexible user sponsorship and join.