To register a third person, you have to add them to your friends under "My account" and "Manage friends". If the person in question has an MSO account, you can find it by entering their name, date of birth and nationality. If they do not have an account, you can create one for them by following the steps below.
You can also add the person directly from the registration page of the event of your choice.
Once this is done, go to the registration page of your event. You will be able to register someone other than yourself by choosing another name instead of your own after "enter a registration for" (see below).