Linguistic conventions that arise in dialogue reflect common ground and can increase communicative efficiency. Social robots that can understand these conventions and the process by which they arise have the potential to become efficient communication partners. Nevertheless, it is unclear how robots can engage in convention formation when presented with both familiar and new information. We introduce an adaptable game platform, SPOTTER, to study the dynamics of convention formation for visually grounded referring expressions in both human-human and human-robot interaction. Specifically, we seek to elicit convention forming for members of an inner circle of well-known individuals in the common ground, as opposed to individuals from an outer circle, who are unfamiliar. We release an initial corpus of 5000 utterances from two exploratory pilot experiments in Dutch. Different from previous work focussing on human-human interaction, we find that referring expressions for both familiar and unfamiliar individuals maintain their length throughout human-robot interaction. Stable conventions are formed, although these conventions can be impacted by distracting outer circle individuals. With our distinction between familiar and unfamiliar, we create a contrastive operationalization of common ground, which aids research into convention formation.