[mzltest]

This happened because you're in a group where inviter is also in that group. Therefore they can add you to their group by searching their name. I believe this is done automatically though.

Anyway, if you set your username, it's available for public to search for it. If you want full privacy then you can turn off your username. But in this case, if you're joining public group, there is still a chance where you will get invited to unknown group.

I just used telegram and wondering did the inviter to be one of your contact to be able to invite you to another group?As far as I know there is no way for a group member to add other member to another group even I was the group admin.
But yes,when someone invites you it is looks like to be automatically joining.

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If I may know further, what kind of hacking they fear? I mean, Telegram is completely safe since it uses phone number for login purpose, so if you have your number then you're safe. If you say you don't want stranger to join your group then deactivate the link and manually add them.

Until now, I haven't get any kind of hacking activites in my account though.

I think they are not mainly expressing concern to platform but to the bots gathering user information or chat info as these bots will listen to /commands and that might mean bots can get access to chat data which might be sensitive (though there is privacy mode---BTW how can group owner/admins activate that for the robots? I know when it was activated in the bot profile says have no access to messages) in  platform is 100% safe and transparent unless you host your own solution.And that's why I suggested if he is still concerned then build one himself.