This is hard, so I'll try to explain what I understand from the
Diccionario Panhispánico de Dudas:
With verbs that imply a preference or need like "buscar", "necesitar", "preferir", "querer", "desear", etc., if you don't use a preposition, it means you are talking about just any person.
"Necesito una novia que siempre diga la verdad" -> You don't need "a", because this is not a specific person, and since there is an implied uncertainty about whom this person may be, you must use the subjunctive. Yet, you must say: "Necesito
a una novia que siempre
dice la verdad", because you already know who this person is.
When you have transitive verbs like "conseguir", "hallar", "encontrar", "llevar", "traer", "contratar", as well as verbs that imply a perception like "ver" or "conocer" and verbs, the preposition may or may not be used, but you may use it as a hint showing whether or not you are talking about a specific person. Same with verbs like "elegir", "escoger", etc.
Therefore, in the TV example, the preposition may or may not be used. If I should justify the use of the preposition, this should be, as you said, because there is a specific group of people from where these volunteers will be chosen.
Side note:
• El candidato esperaba hallar más personas
que pudieran ser convencidas
de estar de acuerdo con él.