If the customers already have a file, you can register the quote in their name. To do this, click on "register", then on "existing customer". You will then need to enter the customer's email address and wait a few seconds. The customer's first and last name will appear and you can then finalise the registration.
If registering an existing customer doesn't work, you'll need to create a new customer file using a fictitious email address. For example, my project number is "1234567", you can use the email address 1234567@voyage.com. This email will only be used to register the project. You can change it whenever you want to send the project to the customer.