In this paper we study the potential and the challenges posed by multi-user instruments, as tools that can facilitate interaction and responsiveness not only between performers and their instrument but also between performers as well. Several previous studies and taxonomies are mentioned, after what different paradigms exposed with examples based on traditional mechanical acoustic instruments. In the final part, several existing systems and implementations, now in the digital domain, are described and identified according to the models and paradigms previously introduced. Keywords Multi-user instruments, collaborative music, new instruments design guidelines.