This article considers the assessment of individuals' contributions to collaborative creative groups. It draws on research into collaborative group composing and its relationship to a secondary school qualification system, the National Certificate of Educational Achievement (NCEA). It explains how a conceptual model of group composing was used to support valid and successful NCEA music assessment. Following this, two Master of Teaching and Learning students explain how they used this model while on school placement to support younger students to assess their own work when group composing. This article aims to shed some light on abiding pedagogical and assessment issues: how teachers might engage meaningfully and collaboratively with students who are working in groups on creative projects, support them in the assessment of their own learning, and then summatively assess their achievement.