WordPress MU

What is best approach of to install a Forum and have the user/pass the same as their WP user/pass?

Integration with bbpress.

Ah, it is automatic with bbpress?

Yes. bbpress is built to integrate with WP & WPMU.

http://bbpress.org/forums/topic/400-new-spammers-signed-up-in-one-day#post-69114

I am using Wordpress MU, the Buddypress Magazine theme

bbpress is not built to, and does not play well with WPMU.
If you're using WPMU, I strongly strongly advice BuddyPress instead of bbPress.

is this true?????

Uh, no. Someone is not aware that Buddypress actually has BBPRESS built in with it.

The spammer problem is hard to diagnose there because they haven't mentioned how they have bbpress & buddypress working together. (or, if they're just using a BuddyPress THEME without using buddypress.)

Tho, you know, turning of signups in the bbpress admin area would work.

or making use of the anti-spam tools such as Captcha images, simple logic Q&A's, displaying an analogue clock and asking the user to type the time in 24 hour format using a question like "what time was it 35 minutes ago?

You know, all the "normal" methods of avoiding spam sign-ups used by bots.

Yes, people sem to be having some issues with splogs on BuddyPress - yet the entire BP site, the testbp site and a few other very large installs have no issues, and they've implemented the steps they keep telling others to do.