Additional Study Information: The purpose of this study is to find out whether delivering SBRT to the prostate, combined with the study drug atezolizumab, is a safe and effective treatment when given in combination with abiraterone, prednisone, and Lupron (the standard treatment for this disease). In addition, the researchers want to find out whether the study treatment works better than the standard treatment alone. Atezolizumab is a type of drug called a PD-L1 blocker; it blocks a protein located on the surface of some tumor cells and immune cells that can act as a brake on the immune system. Blocking this protein releases the brakes, allowing your immune system to function more efficiently to identify and attack your cancer cells. Atezolizumab has been tested in people with your disease, but the FDA has not approved it to treat prostate cancer. The FDA has approved atezolizumab as a treatment for cancers of the lung, breast, and bladder. The atezolizumab used in this study will be provided by Genentech, Inc. Abiraterone acetate, given in combination with prednisone and Lupron, has been approved by FDA to treat prostate cancer that has spread beyond the prostate to other parts of the body. Abiraterone acetate works by decreasing the production of the male sex hormone (testosterone) that causes prostate cancer to grow. The steroid drug prednisone is given with abiraterone acetate to reduce or prevent some of its side effects. Lupron (leuprolide) is a hormone treatment that reduces levels of testosterone, which slows the growth of prostate cancer. The researchers think that adding atezolizumab and SBRT to treatment with abiraterone acetate, prednisone, and Lupron, could boost your immune systems ability to identify and destroy cancer cells, which could prevent your cancer from getting worse (progressing).