Designing a collaborative performance requires the use of paradigms and technologies which can deeply influence the whole piece experience. In this paper we define a set of six variables, and use them to describe and evaluate a number of platforms for participatory performances. Based on this evaluation, the Augmented Stage is introduced. Such concept describes how Augmented Reality techniques can be used to superimpose a performance stage with a virtual environment, populated with interactive elements. The manipulation of these objects allows spectators to contribute to the visual and sonic outcome of the performance through their mobile devices, while keeping their freedom to focus on the stage. An interactive acoustic rock performance based on this concept was staged. Questionnaires distributed to the audience and performers' comments have been analyzed, contributing to an evaluation of the presented concept and platform done through the defined variables.