Many workflows contain activities that are carried out by business professionals using a set of task specific tools. These activities, usually called people activities, are usually modeled as a single task within an overall process model; the actual processing by the business professional however involves in general many different steps that each business professional carries out according to their personal preferences. We introduce in this paper the architecture of a system that helps business professionals organizing people activities. The system provides for the dynamic definition and management of sub tasks, the organization and management of sub tasks as parts of a user-specific process models and the interaction with the workflow management system that controls the process instances containing those people activities. The main contribution of this work is a concept allowing user to refine people activities using personal activity processes which are modeled by users themselves.