When adding an activity, a new contact can be created, but it only asks for name and email. How to configure (add fields)?
Steve welcome to CiviCRM stack exchange. Yes you can add more fields to the form. They are rendered from profile.


Navigate to CiviCRM >> Administer >> Customize Data and Screens >> Profiles
Click on Reserved profiles tab
Find for 'New Organization', click on 'Fields' link 
Add more fields you would like to have. You can only add contact and organization field else the form won't be rendered and through error.


HTH

Pradeep