This study investigates the roles of learner agency and group work in learning writing in English as a foreign language (EFL). Through exploratory and participatory action research, this study examines how learner agency and group work function amidst the activity system of task-based EFL writing, especially how they influence and are influenced by the operations of the components of the activity system and how they impact learners' learning and the outcome of the learning activity. The findings show that the classroom teacher could design a learning activity to capitalise on the potentials of learner agency and group work to enhance students' learning, that the roles of learner agency and group work are shaped by the cultural context in which the learning activity takes place, and that the context dynamically influences the relationships among the components of the activity system.