You will need the Full Write Forms permission to perform this action.
You have control over which people fields on a form appear on an associated event.
If the "Add person to guest list if form is linked to an event?" option is checked, the system will automatically add each person entered in that field to the guest list of the event being signed up for.
Receiving this message (above)? This means that the registration form is not set up to add people to the guest list of this event. You would need to mark "Add person to guest list if form is linked to an event?" option within the Person Profile Information field and/or Special People Field.
People fields that have this option enabled will show as in the above screenshot.
The option is checked by default. Note that this can also be set independently for the submitter of the form. In this way, you can create a form where only the people in the People Fields are added to the guest list of the event. For example, when a parent signs up their children for an event but is not going.