An objective of multi-agent systems is to build robust intelligent systems capable of existing in complex environments. These systems are often characterised as being uncertain and open to change which make such systems far more difficult to design and understand. Some of this uncertainty and change occurs in open agent environments where agents can freely enter and exit the system. In this paper we will examine this form of population change in a game theoretic setting. These simulations involve studying population change through a number of alternative viscosity models. The simulations will examine two possible trust models. All our simulations will use a simple choice and refusal game environment within which agents may freely choose with which of their peers to interact.