Each user has an ID and their email address is the main field on the database to match the ID with the registration. If more than one person had the same email, the system could not identify the right person to assign certain key data (registration form, tickets, emails, etc.).
The main delegates details (First name, Last name, Email address, Company role and Job role) are compulsory to fill so we make sure you capture that data as it's critical to generate tickets, invoices (if any), etc. The way around this is to type a fake email.
You can use the Group Booking system to allow a purchaser of multiple tickets, leave guests with empty details and allow the purchaser to come back in the future and edit the guest tickets.