Although recent research has provided empirical support for the codevelopment of group interaction (defined as development in a common direction of perceptions of group relationships), no studies have yet investigated the "causes" of codevelopment among group members in the long term. The aim of the current study is to examine how the perceptions of an individual group member, and other group members, regarding group relationships influence each other over the course of treatment. We analyzed group data from 168 adult obese patients who participated in (1 of 20) 12-session therapy groups for weight management. Group relationships were measured by the Group Questionnaire (Krogel et al., 2013), which was administered to group members in early, middle and late group sessions. The longitudinal version of the group actor-partner interdependence model was used to analyze the influence among members. Results showed that there is no mutual influence in the codevelopment of both positive bonding and negative relationship. However, positive bonding of other group members in the middle of the group influenced positive bonding of the individual group member in the late group session. Moreover, individual group member negative relationships in early and middle sessions influenced other group member negative relationships in middle and late sessions. Finally, there was evidence of mutual influence in the codevelopment of positive working early in the group. The findings show that the process of influence among members is complex with regard to the specific dimension of the group relationship assessed and the time spent in group treatment. Public Significance Statement This study suggests that there is a mutual influence between the perceptions of the individual group member and the rest of the group regarding group therapy relationships. Additionally, it highlights how an individual group member and other group members codevelop a similar perception of the group relationship. However, the specific type of influence was related to the stage of group development and the specific dimension of group relationships examined.