I think i have found an issue with the module that is only visible on larger sites that are using multiple group entries for the same group in the NIS maps. This comes from the bug that NIS can only handle 1024 chars per line, so if a group has more members that exceeds 1024 chars, a new line is added with the same GID and NAME. yp tools handles this fine, it simply reports several "lines" for the same group. For ex:

> ypcat group | grep FOO | cut -d ':' -f 1-3
FOO:x:17776
FOO:x:17776
FOO:x:17776
FOO:x:17776
FOO:x:17776

when i do the same using the python NIS module it only gives me the users for one of the maps for this group. I guess the "correct" behavior from the module should be to concatenate/append all users from all the maps for each specific group.