Drawing on self-expansion theory, social identity theory, and social influence theory, this study examines how two sources of brand communication - offline friend communities and online communities—shape online purchase intention through self-expansion and brand identification. It also addresses a core limitation of dominant technology acceptance models, which treat social influence as monolithic and fail to capture source-level psychological effects. A quantitative survey of millennials (n = 224) is analyzed using partial least squares structural equation modelling and multi-group analysis. Results show that offline friend community belonging activates self-expansion, while online community belonging drives brand identification—two distinct pathways to purchase. A significant gender split emerges: both mechanisms predict purchase intention for males but not for females. Managerial implications include gender-specific budget allocation guidance between offline referral and online community strategies, and differentiated messaging to activate self-expansion and brand identification respectively.