Objective An individual’s attachment style may impact how they interact with their therapy group. This study examined the moderating role of a group member’s attachment on the dynamic relationships between that group member’s (actor) and other group members’ (partner) therapeutic alliances and symptom outcomes. Method: This is a secondary analysis of data from a trial testing the outcome of emotionally-focused group therapy for binge-eating disorder. The sample consisted of 2,360 sessions nested within 118 group members who attended a 20-session treatment. Patients recorded binge eating episodes (BEE), their body weight and an alliance measure session-by-session. Results Dynamic structural equation modelling showed decreases in BEE and weight over the therapy. When attachment style was not included in the model, higher-than-average partner’s alliance scores in the previous session were related to decreases in BEE in the current session. Attachment style moderated these actor and partner effects. For patients with preoccupied attachments, higher-than-average actor alliance in the previous session was related to subsequent decreases in BEE. For patients with dismissing or disorganized attachments, higher partner alliance in the previous session was related to subsequent decreases in BEE. Conclusion Group members’ attachment characteristics can play a role in the development of alliance-outcome patterns in group therapy.