Is there any REST api available to fetch user/group information from Ranger?
There are, but it seems that they are for Ranger internal use. There aren't any public APIs. Here is the public API documentation for Ranger 0.5. You can also take a look at the following classes if you're interested:
The UI is hitting some user and group endpoints, but it doesn't seem as if there is documentation around them. I wouldn't rely on them.