Redefine the word Guest
Redefine the word GUEST. Guest should be someone who is attending with the Attendee as a Guest (wife, husband, child, etc.) and may participate in all or some of the scheduled events as a Guest at costs different than the attendee such as additional dinners, tours, sports, etc.
If the attendee want to register an additional Attendee it should say “Register an additional attendee” not additional Guest.
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But this exactly what "Guest" currently mean. To register additional person, the one should start registration process again, not using Guest function.
But I do think that user interface can be different to simplify understanding and make it less confusing to adding guests vs. additional registrants.