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.
This can be effective if the group admin can attach this thing. But if the admin has to take so much pressure to manage a group, it will discourage him a lot. Again, it is important to think about creating a group and if someone is a victim of a scam because of that group, he should take responsibility for it. Everyone in the crypto community knows that there will be more scammers in Telegram. So it is difficult to manage a telegram group.
It's difficult but it's possible, at the end of the day, it still depends on how the admin will manage the group, because with an irresponsible admin, the group will not be successful. If there are other groups who are not abused by scammers and spammers, then I think it's not impossible here, the most important is that the pick or the betting tip is accurate, or it wins most of the time.