Managing collaborations during the execution of a process is complex and challenging, especially for managing the collaboration inside a task having multiple instances performed by various actors. Existing process management approaches propose either a rigid control for conducting such collaborative tasks or no control at all. Aiming at a more flexible way to execute and control multi-instance tasks, we investigate a solution based on late-binding mechanism to allow actors choosing dynamically suitable strategies to perform their collaboration. First, we propose a process modeling language which focuses on describing multi-instance tasks and their dynamic instances at execution time. Second, this language is then used to represent patterns capturing reusable collaboration strategies. A prototype of a pattern-based Process Management System has been developed to demonstrate the possible flexible execution of collaborative tasks.