i know its just for fun...girls also dont seem to have any wrong intentions...but is it justifiable?
From an Islamic perspective, as Gaia mentioned above, it's not okay to show certain parts of your body to even the same gender. But to me, I don't think it's a big deal to wear sleeveless and bare a little big of leg (just for the hilarity of it) to an ALL GIRLS party. You could even change into those "fun" clothes when you get to the party....so you're not seen in them outside in public.
You know which girls are similar to you in thinking (the kind that dress decent...are only making this rare change for the humor)...who are not going to abuse it any way. Invite people who you are more compatible with to the party. And there's nothing wrong with letting the guests know about the attire expectations (formal, informal, costume, particular theme, etc).
Gah, I would just be so uncomfortable because of all the stares from everyone else. I'll wear capris, half sleeves, 3/4 sleeves, but nothing less than that. It's just weird wearing something I'm not used to even if at a all women's thing.
Assuming its a question for "Muslim women", as far as within the Islamic limits (sa-tter), they can choose whatever they want to. Some body parts are not allowed to be reveled even in same gender.
I think there's no problem in dressing up in an all girls party (making sure that there's no camera or any pictures been taking), but as mentioned earlier there is still that sharam and haya that girls have amongst each other.
One time, i remember is when i was around 15 or 16, i went to a Muslim girls camp, and we were free to wear anything we wanted as there was no males. Majority of the girls came from a Lebanese background. Me and my cousins still wore clothes that we would wear in the presence of men, but some of these girls wore clothes as if they had specifically bought it for this camp or they had it somewhere hidden in their cupboard
I think it depends how your brought up and the culture you belong to. I have been to many indo-pak female parties where they have assured girls can take of their hijab, but i have never ever felt comfortable to do so. Even when we have dawats at home, men and women are totally segregated i still never felt comfortable to, my mother and chachi always tell us of for wearing it at home in dawats but we still never feel compelled to take it of. Maybe because we are just too attached to it. :)