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.
^ Because Telegram is easy to manipulate and it is easy also to create a bunch of dummies accounts so that your Telegram groups look interesting.
I don't trust the Telegram group because there are a lot of fraudulent activities that possible you will most likely become a victim of those scammers, though I did not say OP belongs to them just awareness. Instead, why not just create your own website, it seems to look trusted if you have a live betting site which everyone could trust.