Some events may allow you to register others and even add them to your household. Look for the options on your signup when registering the attendees in your household.
When registering, if you see the option to Add a new person or household member, you can choose those options to register someone besides those in your family.
When you choose to add someone, you need to enter their first and last name, and you can add their email address.
If you choose to Add a new person, you may be able to add that person to your household as well by checking the corresponding box.