Family and friends have both helpful and harmful effects on adults’ diabetes self-management. Family-focused Add-on to Motivate Self-care (FAMS) is a mobile phone-delivered intervention designed to improve family/friend involvement, self-efficacy, and self-care via monthly phone coaching, texts tailored to goals, and the option to invite a support person to receive texts. We sought to evaluate how FAMS was received by a diverse group of adults with Type 2 diabetes and if FAMS improved diabetes-specific family/friend involvement (increased helpful and reduced harmful), diabetes self-efficacy, and self-care (diet and physical activity). We also assessed if improvements in family/friend involvement mediated improvements in self-efficacy and self-care. Methods Participants were prospectively assigned to enhanced treatment as usual (control), an individualized text messaging intervention alone, or the individualized text messaging intervention plus FAMS for 6 months. Participants completed surveys at baseline, 3 and 6 months, and postintervention interviews. Between-group and multiple mediator analyses followed intention-to-treat principles. Retention, engagement, and fidelity were high. FAMS was well received and helped participants realize the value of involving family/friends in their care. Relative to control, FAMS participants had improved family/friend involvement, self-efficacy, and diet (but not physical activity) at 3 and 6 months (all ps < .05). Improvements in family/friend involvement mediated effects on self-efficacy and diet for FAMS participants but not for the individualized intervention group. The promise of effectively engaging patients’ family and friends lies in sustained long-term behavior change. This work represents a first step toward this goal by demonstrating how content targeting helpful and harmful family/friend involvement can drive short-term effects.