The membership application form is a combination of common fields and membership fields.
All your common fields and membership fields will appear unless they are not enabled for the selected membership level within the field's Use in setting.
So, to remove a field from the membership application form, disable it for the appropriate membership levels within the field's Use in setting.
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.
To add more fields to the form, you create more common fields or membership fields.
For more information on customizing the membership application form, click here.
