I still don't see the reason why. If someone joins your Facebook group, then you can contact them. That was supposed to be the point of the group, and you don't need their name to do so. And if someone doesn't want to join the Facebook group, then your having their name and hull number doesn't do anything. You still can't contact them. That is, unless you later ask for their phone number, address, credit card, social security number and name of their first pet.
The problem of having private info on Facebook is that (closed group) or not, people you don't know will have access to the info and can repost that info somewhere else or otherwise use it however they want.
If someone wants to be mayor of Oliverville, then good for them - go off to Facebook and elect yourself mayor if that's what you need to do. On the other hand, and a bit more charitably, it seems that the people pushing the Facebook group are a few relatively new owners who joined the forum right when it was in the middle of a transition and so maybe got a bad first impression of the place. I'm not an owner yet myself, so count me as one of the scary guests with his (her?) 'complex hidden name'; but I've hung out of the forum for over a year now, and it seems like it does the job it needs to do. So my advice, for what it's worth, is to just chill a bit and if after a while you still feel the need to have your own space, then go for it. But frankly, the fact that you're having to do a little song and dance every few weeks to push for people to join tells me that the need really isn't there.