Can you join the Mythic Dawn in Oblivion

No there is no possible way but when you get to the point where you have to steal the book if you steal it without being caught the people at the base wont attack you but they wont give you any quests or anything.

There is one way that I've found, but I suppose it's not actually 'joining' them. You can permanently be in the Mythic Dawn, but you won't receive any quests. Only thing you get from it are 4 faction related dialogue lines, a disposition drop in the blades, and a disposition raise with the other members of the Mythic Dawn.