Collaborative learning in class-based teaching presents a challenge for a tutor to ensure every group and individual student has the best learning experience. We present Group Tagging, a web application that supports reflection on collaborative, group-based classroom activities. Group Tagging provides students with an opportunity to record important moments within the class-based group work and enables reflection on and promotion of professional skills such as communication, collaboration and critical thinking. After class, students use the tagged clips to create short videos showcasing their group work activities, which can later be reviewed by the teacher. We report on a deployment of Group Tagging in an undergraduate Computing Science class with 48 students over a semester. Through our analysis of interviews and log data, we show that Group Tagging helped the students remain attentive and on-task during group work, and encouraged them to participate more during group activities.