Remember that all people in VSys require a group and a type. While you probably don't want your volunteers picking their own group and type online, you do want to be able to set it for them, and possibly set default values for new people.
In the application designer, place the fields Group (under Person) and Type (also under Person). To make these fields invisible to applicants on the web, after adding the field, right-click on it and select Hidden in web interface. For newly-created people you can right-click on one of these and select Set a default value. New people will get this as their group or type. In either case, when processing application forms, you will see the fields, but the volunteers online will not. Ensure that the field has a value, and VSys will let you save the application.
You can do this to the application design even though there are already pending applications that are filled out and ready for processing. You'll be able to process them and see the field once you've saved the changes here.