Of course, such an event can be done, but almost certainly an invasion of all sorts of smart people and scammers will immediately begin, who will want to troll or even hope to somehow make money on it.
If the group setting is public, then potentially scammers can join at any time and post anything to trap other gamblers or even scam to make money. I think admin should limit new members joining and being active 24/7 to monitoring and verifying news from every poster.
Scammers are waiting for a chance, and they can use it to scam gamblers. So I also agree with your suggestion, if the admin limits the time to join the group members. Then it will be much more effective to deter scammers. It is very difficult to manage a group properly and scammer free. Therefore, the OP should manage this group with all considerations in mind, so as not to create problems in the future.