This study aims to introduce and evaluate a peer assessment method as a complementary approach to Slack-based engagement assessment in face-to-face group work. The participants were students who attended a Japan-Thailand joint global project-based learning (PBL) workshop held in 2024. Posts made on Slack were classified into three types—A, B, and C—according to their level of contribution. Additionally, students conducted peer assessments using a four-point Likert scale to evaluate contributions across three dimensions: idea generation, discussion, and task execution. Multiple regression analysis revealed that the number of Type A posts on Slack had a significant positive correlation with peer-assessed contributions in both idea generation and task execution. However, no significant relationship was observed with discussion. Furthermore, Type C posts had a significant negative impact on task execution. These findings indicate that while Slack-based evaluation has certain validity even in face-to-face settings, it has limitations in capturing oral discussions, which are less likely to be documented.