Background There has been little attention paid to the role of therapeutic processes in group therapy outcomes for cancer patients participating in group. The goal was to evaluate the contribution of 3 group processes—group climate (conflict, engagement, and avoidance) working alliance and therapeutic realizations—to the outcomes of 2 couple‐focused approaches to group treatment. Methods Three hundred and two women with early stage breast cancer and their partners were randomized to one of 2 conditions: an 8‐session enhanced couple‐focused group (ECG) intervention or a couples' support group participated. Couples completed measures of depressive symptoms and well‐being before and 6 months after group. Group process measures were completed after sessions 4 and 8. Results Support group participants (both patients and partners) perceived higher engagement and less avoidance than ECG participants. Conflict, working alliance, and therapeutic realizations did not differ. Group engagement, working alliance, and therapeutic realizations increased, and group conflict decreased over the course of both treatments. Greater conflict was associated with more posttreatment anxiety and lower well‐being, and engagement was associated with higher posttreatment well‐being. Patients whose partners reported higher conflict reported greater posttreatment anxiety. Working alliance was associated with posttreatment anxiety for ECG patients and with well‐being among participants whose partners reported higher working alliance. Conclusions Fostering a positive group environment bolsters treatment efficacy for women with early stage breast cancer and their partners attending couple‐focused groups. Facilitating the leader‐member alliance bolsters treatment efficacy. Improving engagement with one member of a couple impacts the other member.