The problem here is that many Telegram scammers aren't even part of the groups in which they scam. Take impersonators for example, who pretend to be group admins or trusted group members. The moment you join or ask a question, you will get contacted over PM by such people who use similar usernames and pictures that group admins do. They will ask if you need help, if you want to invest, join a special group, etc., etc. Their tactics obviously work good enough, otherwise they would abandon the idea.
This happened to me already, and in my first year in crypto back in 2018, I almost believed a scammer.
The scammer was offering some discount in a private deal for an ICO. I Believe this is how they get most people, offering free money or very good deals and discounts.