It's like the Guild of Light. You cannot join YOURSELF, you will be joined by the questgiver of the starting quest of the chosen path. Magnus wants you to pass Walls of Mists without killing - and then he joins you to the Guild of Light, Archibald wants you to pass Breeding Zone - and then he joins you to the Guild of Dark.Therefore, complete the starting quest, report at Archibald, and check your character screens. You will see that all your guys are members of the Guild of Dark.
As far as the spells are concerned, you will need to visit any of the guild and learn the Dark Magic. Note that only certain classes can learn it and that they always need the second promotion for it.
