Hi Matthew,
This is feature is coming soon - we call it dependant page. Basically is the ability to control the property card of an object based on value. For example different property card for different project types, or in your cased different fields for your external users.
Meanwhile, I would recommend to add those fields a a section in the user level (leave it blank for users that's irrelevant for them), once the feature is out you'll be able to roll it out pretty easily.
Thanks,
Tamir