Though online peer assessment is recognized as a critical factor in enhancing learning performance, pedagogical strategies and analysis of students' peer assessment at the group level, rather than the individual level, are underexplored. Online group assessment (OGA) focuses on assessing peer-group work in an online environment. A total of 64 student teachers participated in this study, where they were divided into multiple groups of four. Each group was required to collaborate on completing an instructional design and engage in OGA activities. We utilized the Technological Pedagogical Content Knowledge (TPACK) scale to assess the instructional designs of student teachers, evaluating their ability to integrate technology, pedagogy, and content knowledge. In this research, we consider the TPACK scores of each group's instructional design as their learning performance. The correlations between providing, receiving, and responding to comments and group learning performance were explored by adopting a mixed methods approach. The results indicated that OGA enhanced group learning performance. Providing comments was more associated with improved group learning performance than receiving and responding to them. Furthermore, providing informative comments was more associated with group learning performance than providing other types of comments. In addition, "innovative" responses were positively associated with group learning performance, while "uptake" responses were negatively associated with group learning performance. Finally, the discussion and suggestions of intervention for different stages of OGA are provided to help design and implement OGA activities in the future.