There is a special field called a Secondary Email Address. Since not every event organizer needs this feature, by default, it is Not Visible and Not Required.

It can be used for the following purposes:

• Someone else registers for the registrant - typical scenario: an admin assistant registers 4 executives using their respective email addresses. The assistant would enter their email address into this field and therefore would receive a copy of every email address sent to the registrant. This includes confirmation email messages, reminders to attend, session notifications as well as any other messages the event organizer sends.
• Registrants need to have all of their email messages sent to, say, a home and a work email address.