You can customize your invitation email in the “Email template” tab in Settings. There, you can set the default email template, edit an existing message, or add a new one.
You can also customize your message when you’re inviting the candidates by clicking on the “Edit mail” button. If you’re making any changes to your message, then you can choose if you’d like to send it to your candidates as a one-time, or save it as a new template.