Approaches to evaluating individual students within the group include student distribution of grade points within a group (peer evaluation), peer evaluation of project tasks and group maintenance tasks (peer evaluation), multiplying the group grade by an individual weighting (instructor/self peer/group evaluation), group grade plus/minus individual contribution grade (instructor/self/peer/group evaluation), shared group grade with instructor intervention where necessary (instructor evaluation).
Richards, D. (2009). Designing Project-Based Courses with a Focus on Group Formation and Assessment. ACM Transactions on Computing Education, 9(1), 1-40. doi:10.1145/1513593.1513595