Argumentative debates are a powerful tool for resolving conflicts and reaching agreements in open environments such as on-line communities. Here we introduce an argumentation framework to structure argumentative debates. Our framework represents the arguments issued by the participants involved in a debate, the (attack and defence) relationships between them, as well as participants' opinions on them. Furthermore, we tackle the problem of computing a collective decision from participants' opinions. With this aim, we design an aggregation function to ensure that participants reach a coherent collective decision.