Buddypress, MU, and mainsite Registration... OH MY!

Dang I'm confused...

Working really hard on getting a student site set up at brainbuffet.net.

Already had to trash it twice because I couldn't figure things out, so it's pretty much a skeleton at this point.

Main problem is that I can't figure out when they sign up, if they're signing up for my site, Buddypress, and/or their blog. There are two username fields when they sign in and I don't know why. They don't need to be members of the main blog, just buddypress and their blog.

The Reg page asks for password but then generates one anyway, and I don't know what password is for what section.

It's all pretty dang confusing. Can anyone help?

feel free to visit brainbuffet.net and look at the registration page. PM if you'd like the signupcode.