You have several options.
The first possibility is to create a new item identical to the item available to all participants but without costs and hidden from normal participants.
Therefore it will be displayed on the registration pages to admin users only.
Choose this option if you plan ahead and if you do the registration in lieu of the invited persons.
Please go to:
Overview => Settings => Manage Events and Items
... and choose the item you wish to create a ‘secret’ option for and select “Copy Entry for New Event/Item”.
Keep all the settings, except:
For the option "Price Category" choose the setting "No costs apply".
For the option "Visual Style of the Event/Item" choose the setting "Hide on registration form (this event/item is neither visible nor available to normal users)".
During the registration process that you carry out as an admin in lieu of the participant, this item will appear, but the item will not be displayed to normal participants.