The opportunity to leverage crowd sourcing-based model to facilitate software requirements acquisition has been recognized to maximize the advantages of the diversity of talents and expertise available within the crowd. Identifying well-suited participants is a common issue in crowd sourcing system. Requirements acquisition tasks call for participants with particular kind of domain knowledge. However, current crowd sourcing system failed to provide such kind of identification among participants. We observed that participants with a particular kind of domain knowledge often have the opportunity to cluster in particular spatiotemporal spaces. Based on this observation, we propose a novel opportunistic participant recruitment framework to enable organizers to recruit participants with desired kind of domain knowledge in a more efficient way. We analyzed the feasibility of our opportunistic approach through both theoretic study on analytical model and simulated experiment on real world mobility model. The results showed the feasibility of our approach.