Get users inside NT AUTHORITY\\authenticated users c#

Question

I want to get all the users inside the NT AUTHORITY\authenticated users. I've spent the whole day looking for answers with no success (I've learned along the way though).

1. It seems that not all AD users are in this group. So, looping through all AD users is not an option.
2. I've tried the snippet from here, but the group variable is null.
3. In SharePoint, I've also tried the SPUtility.GetPrincipalsInGroup, which also returns null.
4. Also tried bunch of other things, which didn't work either. :(

Any help is greatly appreciated.

Update: I did more research and learned that this group is a pseudo group, kind of like a non-existant group. Not 100% sure what's going on. But how can I determine which users are part of this group?