This paper proposes an "Open Collective Robotics" research field, defined as an experimental study of the concepts and techniques that could lead to true collectivities between Humans and Robots. We discuss the kind of social aptitudes required for such robots groups and defend the notions of physical and social situatedness. We also present the MICRobES Project, which is a long-term experiment involving a group of 10 autonomous mobile robots immersed in an environment inhabited by humans.