Despite the needs for the facilitation for multicultural group work (MCGW) in classrooms in internationalized higher education, research on teaching assistants' (TAs) potential to facilitate MCGW has been lacked. This mixed-method study examined how a course design for activating MCGW, which strategically engaged TAs, influenced the social dynamics of MCGW and students' learning outcomes in an English-taught undergraduate course at a standalone international university in Japan. The results revealed that both Japanese and international students made group work inclusive and international students demonstrated altruistic attitudes for Japanese students with foundational English skills. Students expressed their views in their groups and increasingly bore responsibility for group tasks, but not to the extent that everyone in the group equally expressed their views and bore an equal share of the responsibility. Meanwhile, the percentage of students receiving lower grades decreased. Therefore, the course design positively affected the group dynamics and students' learning outcomes.