This article presents a survey of literature in the area of Human-Robot
Interaction (HRI), specifically on systems containing more than two agents
(i.e., having multiple humans and/or multiple robots). We identify three core
aspects of ``Multi-agent" HRI systems that are useful for understanding how
these systems differ from dyadic systems and from one another. These are the
Team structure, Interaction style among agents, and the system's Computational
characteristics. Under these core aspects, we present five attributes of HRI
systems, namely Team size, Team composition, Interaction model, Communication
modalities, and Robot control. These attributes are used to characterize and
distinguish one system from another. We populate resulting categories with
examples from recent literature along with a brief discussion of their
applications and analyze how these attributes differ from the case of dyadic
human-robot systems. We summarize key observations from the current literature,
and identify challenges and promising areas for future research in this domain.
In order to realize the vision of robots being part of the society and
interacting seamlessly with humans, there is a need to expand research on
multi-human -- multi-robot systems. Not only do these systems require
coordination among several agents, they also involve multi-agent and indirect
interactions which are absent from dyadic HRI systems. Adding multiple agents
in HRI systems requires advanced interaction schemes, behavior understanding
and control methods to allow natural interactions among humans and robots. In
addition, research on human behavioral understanding in mixed human-robot teams
also requires more attention. This will help formulate and implement effective
robot control policies in HRI systems with large numbers of heterogeneous
robots and humans; a team composition reflecting many real-world scenarios.Comment: 23 pages, 7 figure