People working with ad-hoc collaboration tools suffer from information overload and information scatter. Our five-month study of project managers found their work comprised of fragmented activities scattering in task, people and application. However, these loosely coupled collaborative activities were implicitly organized by hierarchical activity threads through intrusion and digression, and geared into work breakdown structure (WBS) in high level. Our investigation gives implications on making people’s work manageable as well as flexible, by integrating lightweight collaborative activities with centralized processes.