User registration process in product/edublogs themes

I'm hoping to get some direction on coding/theming to help understand how the /registration process works with the Product/Edublogs and I'm sure some of the other custom themes.

We are working on a multisite server and decided to use the iThemes Builder Theme to give us more flexibility with page layouts/widget areas etc..

It's all be great until we get to the point of trying to direct people to our sign up page at /wp-signup.php.
The page works and it's there but it's "outside the theme" and we want the registration page to show inside the content or we can create a full page widget area if it's easier.

I tried to look at the /registration of the product them and the activate.php and register.php but I'm not that good with coding.

So I'm asking for either:
A-Help understanding the Product theme /registration process to see if I can adapt it to my Builder theme
B-What PHP can I put in my theme or create a template to display the registration page in my theme.
C- Is there a plugin to help with this I'm not aware of? Is there a widget I can drop in a sidebar/page to handle the registration process?
D-Better ideas or solutions?

Thanks again for all the "rookie" help, we are close to roll out soon!