[tiwil]

Hmm... I guess although you do not have an username, you should still have an unique ID as your identifier on Telegram.

And that was why other members on the "malicious" group can still catch you on.

That means, you might just have not set an "easy-to-remember" username yourself. But it does not mean that your profile does not have any identifier on Telegram.

Indeed. Once you met someone in group then your profile is immediately available for them to invite. So as I said earlier, if you want to be safe then you shouldn't join any unknown/public group at all.