Anyway, is It a public group that you have? You should verify your members as well if they are real or not. It depends on what group you have, too, though.
Thanks. I monitor the groups daily and ban people that share stuff like this, report it here exactly because of those reasons you mentioned.Maybe someone will search before "investing" and save themselves that way from loses.
Regarding members I had a bot with captcha to prevent bots from joining, but that way lots of real people get confused and miss the window too verify they are human, so I removed it and I'm back to manual to at least have some kind of organic growth.