Before adding a new person, I issue the viewPerson command to search for them based on email address. Copying the email address of an existing user and pasting it into the command strings below I get back a null set when I should get back the user.
Why check for the user first via the API? As I recall, adding an existing user via the API returns an error message, so it's a non-destructive operation. And if you're adding users via the FogBugz user interface, you already have a very useful search tool available at your fingertips.