In this paper, we propose a novel group mobility model, called Reference Region Group Mobility (RRGM) model, which can be used in the description of group motion behavior as well as individual movement. This model is designed to be applicable to many different scenarios, such as military operation, search and rescue, exhibition hall visiting, building search, etc., where a group may be partitioned into a number of smaller groups and groups may merge whenever necessary. Moreover, by using the density-based approach, our model can control the size of the region to be covered by a group. Our main contribution is on the effectiveness of modeling group mobility scenarios with group partitioning and merging which are most likely to be found in ad hoc networks.