As a Google apps/ Yahoo user you must set up a Zoho username and a password to sign in to the iPhone/Android apps. To do so;

• Visit https://accounts.zoho.com/password (through a web/mobile browser).
• Specify your Google or Yahoo email address in the "Enter your registered email address" field.
  • Please make sure that you enter the email address as 'email[at]gmail[dot]com or as email[at]yahoo[dot]com'.
• Type the characters and validate the "Image verification".
• Click the Request button.
• ou will receive an email shortly to the entered email address with the link which will direct you to set up a new password for your account.
• Then, you can move on to your iPhone/Android device and can use your username(email address) and password to sign in to the Zoho Invoice application.

P.S : If you would like to upgrade your Zoho Invoice account, you can login to the web-app of Zoho Invoice.