AFAIK nzbs.org is closed at the moment and will be for some time. No invites are available. I think it has something to do with migrating to the new server/website...
Don't quote me on this. I'm not 100% positive and i'm not a member either.
That's just what I've read/heard.
As mentioned above, forking out the $10 on nzbmatrix, or nzbsrus, or any of the other sites is well worth it.
I personally have no need or desire to be a member of nzbs.org as I never have any trouble finding what I need