How to customize and add extra fields to the sign-up form

I love the membership plugin, exactly what my client was asking for. I'm pretty sure this must have been asked before but I seem to have a hard time typing in the right search string.

In short, I would like to customize the sign-up page, so new members can fill in all the required buddypress fields before they are moved the the initial level/subscription.

Can you guys provide me with a hint to documentation or where I would need to look in the code to do this?