"They met a lot of new people."
Translation:Ellos conocieron a mucha gente nueva.
This was tricky. I also put nueva gente initially, because gente nueva sounded like brand new people to me---newly created people.
But then I researched it. Nueva gente really means replacement people---and meeting new people isn't replacing all those you met in the past. And in a sense, these people are brand new, in that you never met them before.
Well, steve, I'm not one who "actually knows," but I suspect it's actually debatable. The personal a is not required with vague references to people. (I think Quiero ver un médico is all right, but it has to be Quiero ver al médico, for example.) I guess it depends on whether you consider "many new people" as a vague reference or as specific people. Duo's preferred answer clearly considers the people "specific," but he accepts the "vague reference" answer, as well.