A cavegirl costume is the most popular costume in Britain for hen nights. I think it is a great excuse to dress it something sexy and funny then running around hitting people with bones.
This is a great group fancy dress idea for women. Cave girl outfits are easy to order online so there should be no problem with the whole group getting kitted out in the right gear.
So what is the best way to wear a cavegirl costume? You should definitely wear your hair up and go for a messy look. Think about putting a plastic bone in it for a Flintstones fancy dress look or even some twigs or something.