The membership application form is automatically assembled using all the common fields and membership fields you have set up. To add more fields to the form, you create more common fields or membership fields. The only way to a remove field from the membership application form is to remove the corresponding common field or membership field from your database.
Common fields and membership fields will appear on your membership application form even if the Others access setting for that field is set to No access, and even if that field is set to No access on the Member privacy settings screen.
For more information on customizing the membership application form, click here.