It depends what you want to say. Are you talking about people in general or a specific group of people? You need to tell us the sentence you want to write and the context you want to use it in. Otherwise, it is impossible to answer your question! [ In English, we say that a question like yours is like asking 'How long is a piece of string?". The only answer that you can possibly give is 'Well, it depends!']

Well, I think it depends of the context. If you write about particular people or about people, who were already mentioned in your text you should add "the", that's for sure.