To add questions, text fields, selections, etc. to the member sign-up page, just go to Dashboard > Members > Attributes. Add the attributes, and set them to show up on the registration page, and whether they're required or not.
That's it. No coding!
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Setting up public registration:
http://www.concrete5.org/documentation/using-concrete5/dashboard/sy...
Building a custom registration page:
http://www.concrete5.org/community/forums/usage/creating-custom-reg...
Registrant group addon (free!):
http://www.concrete5.org/marketplace/addons/registrant-group/...
Hope that helps!