Research has shown how the connections between robots' minds, bodies, and identities can be configured and performed in a variety of ways. In this work, we consider group identity observables: the set of design cues that robot groups use to perform different identity configurations. We explore how group identity observables lead observers to develop different mental models of robot groups. Specifically, we make four key contributions: (1) we define, conceptualize, and taxonomize group identity observables; (2) we use Grounded Theory-informed analysis of qualitative data to produce a taxonomy of users' mental models invoked by variation in those observables; (3) we empirically demonstrate (n=166) how variations in observables lead to different mental models; and (4) we further demonstrate how variations in those observables, and the mental models they evoke, influence key group dynamics constructs like entitativity.