Custom signup form for multisite.

What is the best way to have a custom wp-signup.php form? I need to add more fields to the signup form. It would be nice to have those fields on the profile too, but that is not compulsory.

What I want is, a signup form with some additional fields, imagine like the contact form fields, few checkboxes, text fields, I agree to the terms and conditions button. After the successful signup, I want to send an email with all the files filled, like it would have done if it was a contact form.

On a side note, can Contact form 7 be used for signup/register? I know Gravity forms can be used, but that not a solution for me. It has to be on wp.org or a wpmu plugin, and if both of these won't work then some guidance to make a custom form.

Thanks in advance,