Having a disclaimer on Facebook does not stop a lawsuit and the associated costs to defend. You still have to get a lawyer and that costs, even if you win. It is much easier to just get the insurance. Is the lawyer in your club going to defend you Pro Bono if the club gets sued. You can get sued nowadays for anything. Scuba board past history proves this.
anybody can sue anyone for anything. it is rather hard to sue a "facebook group". there are no "leaders", "officers" nor any kind of structure. it is a social group that happens to meet up and dive.