Abstract-Group-level crowd behavior analysis is a new and promising method with important applications for the video surveillance and understanding of crowds. However, a specific definition for a group in a crowd field has rarely been investigated. This paper proposes a complete and reasonable definition for a group in a crowd field and presents a fast and automatic group detection method. First, automatic and fast density clustering (AFDC) is used to find the group core, which is then refined based on the property of coherent neighbor invariance. This detection method is more adaptive to groups with arbitrary shapes and varying densities because the group core is refined with coherent neighbors. Experiments on hundreds of video clips of public scenes showed that the method achieved an excellent detection performance and attractive statistical results. In particular, the number of people in a group exhibits a power-law distribution truncated by an exponential tail; this is significant to understanding crowd scenes and crowd simulation.