At the heart of cooperation between agents we can find the notion of agent modeling. The representation one agent maintains of other agents is indeed essential for social reasoning. In order to cooperate, an agent must know how it may cooperate with each of its acquaintances and such knowledge is tightly connected to the known characteristics of another agent. Agent modeling is even more significant if we consider open systems. In such systems, agents have to update their representation of others in order to learn a description of any incoming agent. In this paper, we propose a model of representation of others based on the external description defined by Sichman. We make use of our model to allow incoming agents to introduce themselves to others and to classify their acquaintances into stereotypes. These two abilities permit us to conceive an open multi-agent system (MAS) keeping the agent integration task distributed among the agents. An application of such a MAS to the building of open distributed information systems is presented.