Lifestyle Intervention for Young Adults With Serious Mental Illness

Inclusion Criteria: * Young adults ages 18 to 35 * Chart diagnosis of schizophrenia, schizoaffective disorder, bipolar disorder, or major depression * Overweight or obese defined as BMI ≥25 * Enrolled in treatment at the agency for at least 3 months prior to study recruitment * Have not started taking Clozapine or Olanzapine antipsychotic medications over the prior 2 months (dose changes are allowed) * Able and willing to give written informed consent to participate in the study or able to assent with guardian consent * Willingness to be randomized to either of the two conditions * Able and willing to attend the weekly 60-minute weight management session, participate in the weekly 1-hour physical activity sessions, and use the Facebook and mHealth (i.e., Fitbits and text messaging) components of the PeerFIT program Exclusion Criteria: * Medical contraindication to weight loss (e.g., cancer, stroke, liver failure) * Medical conditions (e.g., diabetes, angina) that do not receive medical clearance by a primary care provider * Major surgery planned or likely to occur within the next 6 months * Prior or planned bariatric surgery * Use of prescription weight loss medication within the past 6 months * 5% or greater weight loss in 3 months prior to baseline * Currently enrolled in another weight reduction program * Pregnant or planning a pregnancy, or breastfeeding during study period * Cognitive impairment sufficient to interfere with participant's ability to provide informed co…