Task modelling is a widely recognized activity when designing interactive applications. In this perspective, it is the meeting point between various stakeholders. However, most of the automatic environments that currently allow task modelling only support single users, thus limiting the possible interactions and discussions amongst them. In this paper we present Collaborative CTT, a new Web-based multi-user tool for specifying task models. The tool allows several users, who may even be physically separated, to work on the same model at the same or different time. Among its features, the tool includes mechanisms specific for this type of HCI modelling in order to support coordination, communication and mutual awareness among participants. We discuss the aspects we have addressed in designing the task modelling tool, its main collaborative features, and also report on user feedback gathered through formative tests.