In this paper we present a new model for the requirements analysis of a system. We offer a conceptual model defined following a visual modeling language, called dependence networks. TROPOS uses a visual modeling language called dependence networks in the requirements analysis of a system, and in this paper we propose a new conceptual model extending dependence networks with norms. This improvement allows to define a new type of dependence networks, called conditional dependence networks, representing a new modeling technique for the requirements analysis of a system. Our model, moreover, allows the definition of coalition depending on different kinds of networks. We illustrate our model using the scenario of virtual organizations based on a Grid network.