$ curl -X POST -d "username=admin&ui_password=pass&ui_roles=administrator" https://localhost/rapi/system/users
$ curl -X POST -d "email=newemail@domain.com"

label           additional identifying info
username        user name
ui_password     password for logging into the UI
ui_roles roles (comma-separated) that define what privileges this user has (TODO: document roles)
email           user email address