Anecdotal evidence suggests that participants in conversation can sometimes act as a coalition. This implies a level of conversational organization in which groups of individuals form a coherent unit. This paper investigates the implications of this phenomenon for psycholinguistic and semantic models of shared context in dialog. We present a corpus study of multiparty dialog which shows that, in certain circumstances, people with different levels of overt involvement in a conversation, that is, one responding and one not, can nonetheless access (...) the same shared context. We argue that contemporary models of shared context need to be adapted to capture this situation. To address this, we propose “grounding by proxy,” in which one person can respond on behalf of another, as a simple mechanism by which shared context can accumulate for a coalition as a whole. We explore this hypothesis experimentally by investigating how people in a task-oriented coalition respond when their shared context appears to be weakened. The results provide evidence that, by default, coalition members act on each other's behalf, and when this fails they work to compensate. We conclude that this points to the need for a new concept of collective grounding acts and a corresponding concept of collective contexts in psycholinguistic and semantic models of dialog. (shrink)