Is there a way to join this "secret guild" ?? i had to kill a few while working for the commona tong, but when i found out they are are assassins like the Commona Tong only they are cold blooded and kill and steal without the "Imperial sponsored" writs, It kinda piqued my intrests... ;)

If i can join them, who do i have to talk to?

you can't join them. that dark brotherhood doesn't even " exist " ;)