Solution by jQuery
var n = 8 // adjust it in some way
var inputArea = ['<div class="form-group col-lg-'+n+'">',
'<textarea class="form-control" rows="3" id="textArea-'+n+'">',// let's be nice and not use same IDs twice
'Please put the content inside this html tag.',
'</textarea></div>'].join('')
$('.row').append(inputArea);
However make sure that your back end is ready to handle that input.
EDIT: The solution might not be fancy and using clone() is completely fine too. To keep track of ids I would add a simple variable like n that I would increment every time a new input area is added, and then add it to id.
So, init
var n = 0;
In addLine:
n++;
Set the id (doable in addLine too)
$target.attr('id', 'inputArea-'+n);//Assuming that $target is the inputArea in question