Objective To assess whether group dynamics are associated with weight loss, session attendance, and self-monitoring adherence after 6 months of lifestyle intervention for obesity. Methods Women with obesity (N = 125; mean ± SD BMI = 37.84 ± 3.94 kg/m2; age = 51.99 ± 10.81 years) participated in a 24-week group-based lifestyle weight loss intervention and achieved a weight loss of 9.13 ± 7.15 kg after 6 months. Participants reported their perceptions of group conflict, avoidance, engagement, social support, and attraction at the end of treatment. Multiple regression with forward selection assessed which group dynamic variables were associated with weight loss, attendance, and adherence. Results Greater perceived group conflict was associated with smaller weight losses (β = 1.833, P = 0.044) and lower attendance (β = -2.313, P = 0.002) and adherence rates (β = -2.261, P = 0.030). Higher group attraction was associated with higher attendance rates (β = 0.051, P = 0.039). The association between perceived conflict and weight change was mediated by attendance and adherence (P = 0.019). Conclusions Findings demonstrate that group dynamics associate with weight loss outcomes, attendance, and adherence. Addressing conflicts and fostering acceptance among group members may promote success in group-based lifestyle interventions for obesity.