"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.
Just realised that's more like "met up with", as if they'd already met up before.