The group needs to be active. This is possible only through active conversation and discussion. Initially OP can search and share some articles through which participants will learn about different airdrops and other giveaway if he isn't able to conduct some events on prediction and give rewards. Then onwards depending on the valuable information shared the participants will increase. For now we've got more such groups, so people will look for something unique before joining.
It all depends on the numbers of telegram users he has and the kind of sports he is into to keep the group more active. If he only concentrate on football only, he might have few interested users but if he forcast on so many sports and do have a positive results then more users will join the Telegram group because people will like to invite there friends to benefit also from the game predictions.
To predict football matches or other bets is not that easy and op will need to people too that can bring there predictions to the group so everybody can share there own opinions.
Once there was a telegram group that I joined. It has lots of members but this is useless since the group is still quiet and most members are inactive. There is also one group with lots of members but only those who actively talks are bots. Maybe they put that on purpose or to make the group active but it's kinda annoying since the flow of their convo is kinda obvious uninteresting.
I better be in a group with less members but the talks are genuine. Op didn't specify a game but all forms are allowed as long as it's still gambling. That should make this group big and more talkative but first op needs to do some kind of a lure like what @rmoero121 suggested.