A new Workflow Management System (WFMS) model is presented, that uses a Trust Establishment framework. This new model enables creating dynamic user-role assignment where not all users are known in advance. Thus it can fit into dynamic environments where new users are added, or credentials of existing users are revoked, like on the Web. The model is composed of three distributed agents called Credentials Collector, Role Manager and Task Manager that communicate with each other. The Credentials Collector is responsible for collecting all the needed credentials in order to allow membership of a user in a role, the Role Manager is required to find a suitable user-role assignment which satisfy role assignment constraints, and the Task Manager has to find an assignment of users/roles to tasks which satisfy the workflow constraints. The agents use constraint processing to solve their respective problems, and also attempt to achieve an optimized solution1.