When logged into your Billetto account, you can download your guest list at any time by clicking on My events at the top left of the page and then clicking on the Manage option next to the relevant event. On the left hand side click on "Guest list & reports".
You will need to open the XLS or CSV format of the file to see a guest list which includes the email addresses of all attendees.
You can also look up individual guests and their email addresses under Orders. Here you can also request a refund for guests’ tickets, resend tickets to guests or forward tickets if one of your guests has entered a wrong email address.
Please note that due to new GDPR regulations if you wish to use guest email addresses for none vital reasons you must first gain consent from the individual. For more information see this article: GDPR Compliance