For simplifying billing and limiting data usage, Family Shared Plan (FSP) is now a popular plan provided by telecommunications operators. Based on the 3rd Generation Partnership Project (3GPP) specifications, Online Charging System (OCS) has to determine the Granted Unit (GU) and create reservations per session before a service is delivered by the serving network to a User Equipment (UE). However, fixed GUs cannot adapt to dynamic consumer behaviors, which usually require dynamic GUs. Clearly, giving small GUs will incur too many signaling messages for making reservations, but giving excessive GUs will cause unbalanced distribution of resources, especially in FSP cases. How to determine GUs is an open issue that can be self-defined by telecommunications operators. In this paper, we propose a new scheme for dynamically assigning GUs to UEs belonging to the same FSP, based on their historical data usage and total monthly data allowance. Simulation results show that our scheme can substantially save signalings by at least 22% compared to fixed scheme under unpredictable behaviors.