I'm 95% certain that "group" in this case can be read as "one or more". It should be easy enough to write a test for this. Harder but also possible is to crack open the java source and see what it's doing.
The idea here is you can give the lock to 1 writer or 1+ readers at the same time. It's just trying to say that if there are multiple readers waiting before the next writer, they all get the lock at the same time. This is safe because they're just reading.