Who To Invite To Bachelorette Party

I have heard that it`s ok to invite people that are not invited to the wedding to the bachelorette party, but not to the shower. And I have heard it is not proper to invite people who are not invited to the wedding to either. I have been to bachelorette parties where people who were not invited to the wedding were there buty they were not invited to the shower. Does anyone have any insight on this?