Building trust in a group is not easy. this needs some process to invite more members into groups, moreover members who are actively playing gambling.
Additionally, as an owner of the group, you also need to provide more info in order to make the group live, although maybe some of your members are still silent, they may sometimes open your group and read the information.
Actually, when your group is new enough, you may be far from expectations, not many real members may be interested, but this is the process.
btw, in my opinion, sharing about gambling will also limit some discussion on general strategies or tricks. But you may not find professional gamblers who will share their exact tricks and strategy in your group, none will tell their secret. I think that if your group is active and good enough, the group will be much more appropriate for many new gamblers.
I think that the main problem is that the OP even doesn`t know for what purposes he wants this group. It is possible to create group for just talking - but there are lots of same groups. May be he wants to create group with predictions - in this way he has to search members and test their predictions for some time. Or he wants, as he says, to find members, who thinks the same, but it even sounds strange - the easier way to find group with such participants.