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 [9] uses dependence networks in the requirements analysis, in this paper we propose to extend them 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 the system. Our model, moreover, allows the definition of the notion of coalition depending on the different kinds of network. We present our model using the scenario of virtual organizations based on a Grid network.