I'm just collecting their usernames, nothing more.
Chats are public and their usernames are public too.
Collecting data doesn't matter. But why would you sell these data? It can be seen in various spamming groups without permission which is very annoying. Many scammers buy these data and add users to the shit ico group without permission.
I collect @username of all chat users, in the future this user database can be used for mailing / inviting.
What is this?
I repeat, username is not private information.
"add users to the shit ico group without permission" - this is telegram feature that you can add a user in group without his permission. if you do not like this - write in support of a telegram