This research, focused on small work groups, proposes to examine the role of the leader in the appropriation of collaborative information systems. It is based on a three-year action research conducted during the implementation of the Microsoft Teams collaborative platform at a university. The aim is to understand how groups of students organize themselves to carry out complex projects over a long period of time and with varying distance constraints. Differences in the appropriation of the new tool appeared depending on whether the mode of work was cooperative or collaborative. To understand these different dynamics, we mobilize Vygotsky's activity theories (TA), more precisely Engestr{ö}m's 2nd generation TA model (2000). The results reveal the role of leaders through the cooperation versus collaboration tension. In teams, a leader emerges who organizes cooperation but slows down the appropriation of collaborative features of the new information system.