Does the Android operating system have files like /etc/passwd, /etc/shadow, and /etc/group?

If not, how does android determine whether a user belongs to a certain group?

This thread discusses how that is done: http://groups.google.com/group/android-ndk/browse_thread/thread/adddb27c1a5438e9

It contains a broken link to a git repository though, you can find a list of name and IDs in https://android.googlesource.com/platform/system/core.git/+/master/include/private/android_filesystem_config.h.