Collaborative writing tasks are common in multilingual university-level writing-intensive classes, but how multilingual language learners (MLLs) are socialized into this group work as a discursive practice is still poorly understood. This case study of adult MLLs in multilingual writing classes at a large public university provides insight into how this socialization can conflict with teaching goals. Membership Categorization Analysis (MCA), a micro-analytic approach, is used to investigate how students inter-subjectively assemble categories of identity in order to articulate their understanding of collaborative writing tasks and how they are obligated to negotiate them. Analysis of classroom talk reveals a conflict between student articulations of how to perform writing tasks when they interact with one another as group members and when they interact individually with instructors. Implications and a possible pedagogical intervention for the framing of collaborative writing tasks are discussed.