Desculpe is used for "I'm sorry", it's a not so emphatic way of saying (me perdoe - forgive me). It's also commonly used with the reflexive pronoun: desculpe-me.
Excuse me would be better translated by "com licença", "me dê licença por favor". (literally give me license - to sit, or to leave, or to ask the person to stand aside so you can pass)
By the way, normally "desculpe" (desculpar) should decline, so I would write "desculpem-(me), meninas" (and it's indeed imperative)
You're right, meninas is not the subject. But, for that reason, "Excuse me girls" is a wrong translation, since girls is the subject of that sentence. A verb must always concordate with the subect, therefore "vecê" is the subject -elided in this case-, and meninas is a direct object. The right translation is "Excuse the girls", in which you is the elided subject, and which, by the way, was my answer and was considered wrong by duolingo...
Edit: I'm wrong. That would be "Desculpe as meninas". I still think "desculpe meninas" is not correct, it should be "Desculpai meninas" or "Desculpem meninas" -altough it would be weird to use the formal "Desculpem" when talking to a bunch of girls-.
Yes... "Me desculpem, meninas" (forgive me, girls). I am unsure if "desculpe, meninas" is correct actually, now that you mention it, since it is a plea to a group (2+ girls). I think I have heard it that way a few times... which doesn't mean it is correct Portuguese. I could be wrong though. =]
I have to agree, "desculpe meninas" is just plain wrong in my opinion. "Desculpe" is formal and singular. In this case you're asking for the forgiveness of the girls (plural) hence the verb has to match the subject "desculpem" which would be right if you stop there (imperative). But adding the noun "girls" requires you to be clear about whom are they forgiving as it is a reflexive verb hence "desculpem-me"(Portuguese /PT) or "me desculpem" (Portuguse/BR). Hope I did help...