This paper presents a methodology for the Model Driven Engineering of complex, multi-actor business processes, mixing tasks executed by humans and by machines. The idea is to enrich business description languages with a few extra details on task assignment, semantics, and typed dataflows, so as to enable a two-step generative approach: first the Process Model is automatically transformed into an Application Model, which seamlessly express both human- and machine-executable tasks; secondly, the Application Model is fed to a state-of-the-practice Web/Web Service development tool capable of producing the code. The resulting method and generative framework unify Web Service orchestration and Web User Interface design into a coherent Model-Driven Engineering process.