Answer by WheresAlice for Tab Autocomplete for New User

On my system tab autocompletion is added by:

if [ -f /etc/bash_completion ]; then
    . /etc/bash_completion
fi

in .bashrc. Bashrc doesn't get created for new users since it doesn't exist in /etc/skel. Create .bashrc in /etc/skel with the above code in and it will work for new users. Then copy the file to the new user you just created so it works for them too.