Using Conversation Analysis, this study describes how ‘institutionality’ is accomplished in talk-in-interaction by analyzing how the Korean student group members construct themselves as ‘an institution’ through decision-making. Most conversation-analytic research on institutional talk has been of occupational settings. This study, with data from a voluntary student staff group whose meetings are sporadic and without formal phases, illustrates that the group members’ interaction reveals how they construct themselves as a decision-making group whose members embody different social roles, and ultimately as an institution. Two significant practices are discussed. First, the data show that the members actively search for precedents, which later become the most crucial basis for their decision-making. Second, as a strategy of gathering power over others within their institutional boundary, the members frequently depart from the preference structure of ordinary conversation. Overall, this paper contributes to a better understanding of institutionality with data from a quasi-institutional setting in the relatively under-examined language, Korean.