Aim: The study aimed to investigate the impact of socio-demographic similarity on the probability of attending an adequate dose of a psychoeducational group intervention (≥4 of 6 sessions). Method: The sample comprised 2071 patients (63% female, 93% White, 15% unemployed, mean age 43) who received the Stress Control intervention in the UK’s national Improving Access to Psychological Therapies (IAPT) programme. Similarity indices were constructed to measure each patient’s similarity to the rest of their group on four characteristics: age, gender, ethnicity, and neighbourhood deprivation (Index of Multiple Deprivation; IMD). Results: Multilevel analysis found that patients with greater IMD similarity to their group had significantly higher probabilities of attending an adequate dose of intervention (p =.026, controlling for absolute IMD). A cumulative effect of age similarity, ethnic similarity, and group size was also found, such that patients who were similar in age and ethnicity to their group had higher probabilities of adequate attendance in larger groups (p =.006). Conclusions: These results suggest that socio-demographic comparison (a.k.a. relational demography) may consciously or unconsciously impact on patients’ attendance at group psychoeducational interventions, particularly regarding indicators of socio-economic similarity. Clinical implications include structuring group composition and/or intervention content to maximise attendance and therefore clinical effectiveness.