Without signing up for one myself, I’ve tried digging around online for reviews or complaints from people who have joined any of them. I never found anything substantial, other than a complaint from one of their promoters. It seems they operate on some sort of referral program where people get paid by signing up new members. You may have seen their online promoters on social media, who typically use an image of an attractive naked woman. These people create fake Twitter accounts and Facebook profiles just to promote these shady websites. They can also be found spamming the comment sections of other nudist websites and blogs.
I’ve heard a few strange stories here and there from people who’ve tried them, and none were positive. Most likely, the members you’re seeing on their sites are not real. I think the whole point is to get people to buy a membership (I know one person who signed up for $30/month) and that’s how they scam you.