On my telegram account, I don't know how some group admins manage to get my user name to add me to their group because each time I open my account,
They managed to add you up from a group you joined. It is obvious you've not set restrictions on your telegram account. You have to restrict people from adding to you to certain groups without your permission. Also, restrict anyone from sending you PM through a public group you joined. And lastly be careful in groups you joined, many are full of scammers.