[tiwil]

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.

I'm a little bit miss here. Inviter doesn't need to be in our contact (phone contact) to invite us to a group. They simply will search for your profile and boom, you're in.

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.

Group owner cannot activate privacy mode for a bot. You can check if it's in privacy mode or not by simply checking if he can access message or not (as you said). If not then privacy mode is activated.

In order for bot to welcome a user, its privacy mode must be turned off. So I guess if you're afraid of info stealing, better double check the bot (make sure to use bot with high reputation) or just build your own bot from scratch. It's easy and fast, you can do it with Python