In private Groups, only members will see who else is in the Group and what they've posted. Admin will be able to control who discovers the Group, though, either making it "hidden" or "visible." The same Community Standards will apply to public and private Groups, as they have to public, closed and secret Groups, so Facebook will still be able to monitor for "bad actors and bad content."
The change is in line with the vision Mark Zuckerberg described at the most recent F8 developers conference. Zuckerberg positioned private Groups as a big part of Facebook's strategy going forward, and this seems like a step toward that future. Unfortunately, while the new terms could help clarify who sees which content, the change does little to address Facebook's problem of hate-filled Groups.