Is this legal?
Both MN and WI have State Laws limiting nudity to private locations and facilities where no one who could be offended is likely to come upon it and when it is part of a bona fide dramatic performance. Local ordinances can impose stricter guidelines, such as preventing businesses from allowing nude dancers or nude clientele. Some areas are covered by Federal Law which usually has no specific anti-nudity restrictions but it is up to local government to determine specifics and this can vary widely.
What does this mean? In general, it is LEGAL to be nude, alone or in a group, in a private dwelling or on private property unless others can see it from other property. Most businesses may rent facilities to private nude groups, again, as long as they can not be seen outside the establishment. And, unless specifically set by local government, most Federally owned property and facilities are theoretically able to be used nude. For example, Minneapolis-based nudist groups regularly use portions of the upper St. Croix Scenic Riverway for nude canoeing. Sadly, even where it's legal, nudists are sometimes ticketed and must prove this in court. Please be aware of the law, be careful, and contact an attorney before taking any action.
A WARNING about SEX: Many areas that get nude use are monitored closely or closed because of sexual activity, not the nudity. Public lewd behavior is never appropriate at nude sites and facilities and can wreck it for everyone.