I was a member of some of those secret groups, and somehow they've all been found out. Some people were thrown in FB jail. Consider this: if you're a member of a secret group, found out, thrown in FB jail, how do you explain your absence to regular friends, co-workers and such?
FB jail bans people for other reasons too
besides nudity.I've had friends thrown in jail for posting something political and another for a joke . In terms of the secret groups because I've had 2 .....you really have to vet the members and make sure everyone is on the same page.There are those (usually male) who have fake profiles and join the group for self gratification and not for the true essence of nudity or naturism and when they get called out, they report the groups. I have a group running now with a limited membership , for folks I've gotten to know well ....we post what ever we like and it's been running incident free now for 3 years. I suggest doing something similar with folks you're sure about.Hope this helps . :)