Robots are acting as housekeepers, fitness coaches, tutors for children, companions for the elderly, and more. With more robots entering our lives every day, many interactions will include multiple robots and/or multiple humans – yet most human-robot interaction (HRI) research focuses on a single human interacting with a single robot. Studies in social psychology (e.g., [1,2]) and HRI (e.g., [3-7]) indicate that inter-group interaction varies crucially from inter-individual interaction. First, groups modulate the effects found in dyadic HRI – for example, inter-group interactions are often more aggressive and negative than inter-individual interactions. Second, groups introduce variables that are not possible to study in dyadic HRI – for example, the cohesiveness (entitativity) of an outgroup increases perceived threat from them. Third, group interactions between people and robots require different technical solutions to problems of perception and interaction (e.g. [7]). To have a better understanding of HRI as it will occur in real life, it will be important to enhance our understanding of the specific effects of inter-group interaction depending on situations and robot characteristics. This can also inform robot design, including the development of appropriate interaction frameworks and modalities, robot appearance and autonomous capabilities, and functions of robot groups in everyday human environments.