Admins never DM first, never ask for money, never ask for private keys and so on. Remember this principle and people who know about this as well as are careful enough will not be scammed by impersonators.
Another thing: The admins have at least one group in common with you. The one you just joined. If you click on any Telegram profile, the user info window will pop up. Towards the bottom you will see if you have any groups in common. If you click on that, it will show you which groups. You won't have the group in common you just joined with a scammer because he isn't a part of it. He just follows what's being written publicly and contacts those he believes he can scam.