"See you tomorrow, girls!"
Translation:À demain, les filles !
When addressing a community, you are in the same case as that of a generality, ie, you need the definite article.
To expand on Sitesurf's comment.
Le/ la/ les = the = definite/ particular - that one right there
un/ une/ des = a or some - indefinite/ limited = not any one in particular - some, but not all
In English when we want to refer to a complete generality we just drop the article completely. EG: I like bread = I like all bread, the idea of bread. Bread in general. Not a particular bread, not some bread but all bread. All examples of bread.
However, French requires a modifier, of some kind, where English doesn't. You can't just have bread sitting there in the sentence all by itself. To deal with the need for an article that expresses generality, the French, instead of developing a completely new article, just assigned the role of a generalizing article to le/ la/ les.
Therefore the definite article le/ la/ les has a dual role of indicating something in particular and also in general. Thus le/ la/ les is used when referring to all examples of something.
To determine which of the two meanings is being used by le/ la/ les you need context.
In this example, you can assume that when he says see you tomorrow, girls he is including all of them. He is speaking to all members of that community or group. All examples of something. Therefore, le/ la/ les is required. If he was speaking to a particular, one of them, (which context would have to indicate) then le/ la/ les would be required. If he was speaking to some, but not all of them, then des would be appropriate.
Hope this helps.
always use le/la/les in French while in English we could use only "girls" for short