All of the hen nights I've been too, including my own, we decided that the only person to be dressed up was the hen!
(this was pay back for the amount of times I'd organised someone else's night!!)
If your friends are happy to dress up as well, that's all the better, but I knew that it was going to be me on my own!
We went to a Greek restaurant, and there were 2 other hen nights there too, so that made for an excellent night!!!
HTH
Lo x