Unlike public ones, the community group is not searchable and you'd need an invite link or be invited. I'm not sure why they opted for a private one as well. Publics would have better visibility and much easier for people to join. But they could always change it.
It is very possible that it is a close group for the now so that you can keep it controlled and control the people who join. If they leave the group public when the management is not ready to devote full attention to it, the group can be flooded with scammers who may be looking to look for victims from the group chat and that can end up bringing a bad name to the company. I'm very sure that the company knows that a public group is better and later in the future will have the intention to open it up.
.that's a really good insight and a possible reason for their actions, the reason why I say this is because attention and focus is needed whenever it involves group chat and most especially when it has to do with gamblers. Aside from scammers, inpatient gamblers can cause lots of damage to the casino reputation especially when they ain't that active in terms of response and attending to issues concerning the casino.