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Having it e-mail you all the usernames and passwords is a very very very bad thing out of a security and privacy point of view. Passwords are private. I would never sign up on your site knowing that you would do that.
Now, having said that let's get clear what you want. You want people to be able to access certain parts of the site if they are a member of the forum. You want them to be able to use the same login as on the forum. Let's make it better by saying that you can use the login of the forum, no need to enter your password twice! Just take a look at the SSI Faq's. There is a topic stickied here in this board.
ok, this isnt a "1337" speak forum. Im sure you are capable of typing sentences that I wont need to "decode". Second off, stop bumping posts, it's rude.
If i understand what you want...you want to know members passwords. You cant. They are hashed. You could by installing the smf board on your computer and having the "forgot password" function email other users passwords to you.
EXACTLY like Elissen said, none of this is nessesary. You can use ssi for registration, log in, and permissions. Look at the basic ssi faqs.