This paper presents results obtained from ambient vibration measurements using a remote monitoring system (RMS) on a grandstand in Bradford, United Kingdom. Using this RMS, output-only vibration response data were acquired when the stand was empty and when it was occupied during football and rugby matches. For the occupied stadium, modal properties that corresponded to different crowd activities (sitting, standing and jumping) were also extracted. The configurations of the crowd occupation during sporting events were monitored by correlating vibration response data with synchronised video images. Modal parameter estimations were made on data that were acquired from several events, hence improving the statistical reliability of the estimates. The observations showed that seated crowd occupation led to a decrease in natural frequencies and an increase in damping ratios. Further decreases in natural frequencies were observed when the crowd stood and/or jumped following the scoring of a goal. These observations show that crowd occupation does not only contribute mass to structure, it acts as a dynamic system that interacts with the structure it is occupying. Furthermore, it has properties that vary according to the crowd configuration.