Nope, that is ungrammatical. To be singular, the german sentense should read "She isst mit dem/einem Mädchen" to indicate there is only one.
Just like in English, plurals can be used without an article: "I like fruits".
i was suppose to translate it. I think that it all depends on context. I'm sure that there is something in there that indicates what is going on, but i am too inexperienced with german to know what yet.