The attributes or fields will have to be filled out for each registration that is selected. If 5 registration units are selected, then the attributes will need to be selected 5 times.

One alternative is two create multiple event registration units - 1 for the main registrant and 1 for the guests, which would allow you to make the attributes available on the main unit, but not on the guest unit. You would have to instruct users to select 1 for the main registrant, and then select the guest units separately.