What happens when I use an email address which is not associated with G Suite/Office 365?

When you try to use a different email address, and if this email address is not added as a user in your Zoho Invoice organization, you will be redirected to a page which would look like this:

If you click on Switch Account, you will be re-routed to a page wherein you will be asked to create a new organization.

To avoid this, you can invite the new email address as a user into your Zoho Invoice organization, and set up the integration by logging in as the user.

To know more about the Zoho Invoice - GApps integration, click here.