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.
So in addition to limiting group members, it will be very effective if new members register a telegram account name on the google form that has been provided along with photo evidence of gambling activities to verify active gamblers and not scammers.
I think many channels offer that, but most of the groups that I see are either paid or some special invite necessary to access it. Paid groups are somewhat sketchy because of the "claims" they have, and that's just the tip of the iceberg when it comes to produced stuff. I think another betting group or something is also reselling it. Repeating signals all over, and I think I saw that, but that's mostly with signal groups for trading.
Thanks for your invitation. I do think it's nice to have a place where you can share bets or something.
The risk of the group being paid because the user will share the signal or anything on another private group with the same access and payment method, there is no denying that maybe 20% of the VIP group members do that to replace the money paid to the main group. But basically we hope that scammers are not allowed to share false information with groups about gambling.