When conducting group work, there is always a danger that certain students might take a bit of a back seat and let their teammates do all the work.
To avoid this, provide the students in advance with a ‘peer assessment’ slip. At the end of the lesson / project, each student should complete the slip by dividing 100% between the members of the team to reflect their contribution to the task, and explain their reasoning.
These can then be collected by the teacher. From my experience students tend to do this very fairly and there is little disagreement between the judgements of different team members.