Brief Summary: This phase I/Ib trial investigates the side effects of CC-486 and how well it works in combination with lenalidomide and obinutuzumab in treating patients with CD20 positive B-cell lymphoma that has come back (recurrent) or has not responded to treatment (refractory). Chemotherapy drugs, such as CC-486, work in different ways to stop the growth of cancer cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Lenalidomide is a drug that alters the immune system and may also interfere with the development of tiny blood vessels that help support tumor growth. Therefore, in theory, it may reduce or prevent the growth of cancer cells. Obinutuzumab is a type of antibody therapy that targets and attaches to the CD20 proteins found on follicular lymphoma cells as well as some healthy blood cells. Once attached to the CD20 protein the obinutuzumab is thought to work in different ways, including by helping the immune system destroy the cancer cells and by destroying the cancer cells directly. Giving CC-486 with lenalidomide and obinutuzumab may improve response rates, quality, and duration, and minimize adverse events in patients with B-cell lymphoma.

Detailed Description: PRIMARY OBJECTIVE: To assess the safety and toxicity of oral azacitidine (CC-486) in combination with lenalidomide and obinutuzumab. SECONDARY OBJECTIVES: I. To evaluate the efficacy of CC-486 in combination with lenalidomide and obinutuzumab in subjects with relapsed/refractory indolent B-cell lymphoma as assessed by: Ia. Overall response rate: complete response (CR) + partial response (PR) per 2016 Lugano criteria and Lymphoma Response to Immunomodulatory Therapy Criteria (LYRIC) criteria. Ib. Duration of response (DOR): will be calculated from time of initial response assessment demonstrating at least PR until disease response assessment that demonstrates progressive disease. Ic. Time to response (TTR): calculated as time from registration to first disease response assessment that demonstrates at least PR. Id. Progression-free survival (PFS): Patients are considered a failure for this endpoint if they die or if they relapse/progress or receive additional anti-lymphoma therapy. Ie. Determine the recommended phase 2 dose (RP2D).