I don't know for sure. I was going to ask Gehaktbal but it is around 1:30 AM here so I'm not sure whether he is still online. What he told me is that you need to receive a 'so-called' store invite from a friend to get access.
^ Agreed. Since the release of DOA5 they added eight new ranks (LR, U-, U, U+ and five star ranks). That's way too much imo. Will also stay away from ranked matches probably, because they're a living hell to both search for and play in.
I agree with Goarmagon. There was no reason to add a Rank higher than S+. About 95% of the people on Rank are ass or laggy anyway.