Details are scant as not even the staff has got it's hands on the engine yet, and Timmy is working his pointy helmet off getting the last bits in place
You'll need to register for a faction in order to participate; as it stands, sign-up will probably be opened to Empire (including variants), Kislev, Dogs of War and Dwarfs. You can play
against whoever you like, though battles versus participating races will earn you bonus points.