Yes, I encountered that a lot. Someone adding me for no reason.
I just delete the group whenever I see one that I'm not familiar of.
I haven't change my settings as I'm too lazy. Thanks for that reminder.
It's good if you care about that and immediately go out and delete the group, but most people don't care about it, they just turn off the notification tone, or because they not active anymore in the telegram, but clearly they were put there unofficially.