Administrator Add to Registration is not the registrant's manage page

Open Events → Events → select your event → Registrations.

The administrator registration detail exposes Add to Registration for authorized users when the registration is not marked not-attending. The registrant's My Registration page instead shows existing guests, activities and add-ons; the guest-edit setting controls editing existing guest information there. Do not find an administrator-only button or promise that enabling guest edits adds the same capability.

Have the organizer use the authorized existing-registration addition workflow when appropriate. If the registrant already created a second registration as a workaround, reconcile both registrations and associated tickets/payments before deleting or refunding anything; a second registration is not a safe universal way to add a spouse.