"They have important conversations."
Translation:Ellos tienen conversaciones importantes.
No, ellos still refers to a person (multiple people). It has to do with the role the noun plays in the sentence. If you said, "Ellos ven muchos libros," Ellos is the subject of the sentence. When a person or persons are the subject (the person doing the action in the sentence), you do not use the personal a. But in the sentence, "Yo veo a ellos," ellos is now the direct object (the person or persons receiving the action - in this case, they are the ones being seen). Because ellos is used here as a direct object, you must use the personal a.