I must have missed it cause I read the rules and now I can't seem to recall. I know its something I just can't find, so I thank you in advance for the help.

Typically, you'll get offered a membership when you have an encounter in the right location. Other effects, such as corruption cards, can cause investigators to become members if they're not already.

Thanks! I didn't read all the encounter cards, as I still want to be surprised when I draw them during the game. Thank you for your help.

I use a house rule that if you defeat a cultist, you can take his robe and become a member. It's silly, but fun and acelerates the game.

You can only join the cult through encounters. If you're willing to add custom material to your game, you may find this card useful though:

Well, I guess the correct title for this thread should have been "Why do you have to become a member of the One of the Thousand Cult" ::laughter:: but anyway, yeah, there are no place where you can get a free membership, only encoutners, a la Silver Twilight Lodge. With the only difference that the chance of getting a membership are very thin if you play with all expansions in (those encounters become hyperdiluted)

But, as Jake said, if you don't mind playing with some custom material, you can use the card Jake suggested, or you can use my Custom Institution ( here ), the Church of the Black Goat.