Follicular Lymphoma (FL) is the second most common B-cell cancer and the most common type of cancer of lymphocytes. Unfortunately, this disease is incurable with conventional treatment and the disease recurs in almost all patients. This study will assess how safe and effective epcoritamab is in combination with lenalidomide and rituximab (R2) in treating adult participants with relapsed or refractory (R/R) FL. Adverse events and change in disease condition will be assessed. Epcoritamab is an investigational drug being developed for the treatment of FL. Study doctors put the participants in 1 of 3 groups, called treatment arms. Each group receives a different treatment. Enrollment to one of the groups is closed. Around 500 adult participants with R/R FL will be enrolled in approximately 300 sites across the world. Participants will receive R2 (375 mg/m\^2 intravenous infusion of rituximab up to 5 cycles and oral capsules of 20 mg lenalidomide for up to 12 cycles) alone or in combination with subcutaneous injections of epcoritamab for up to 12 cycles (each cycle is 28 days). There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.