Jquery :: Populate The Input Textfield?

I have a recipients input textfield called 'recipient', which takes usernames separated by a comma. Next to this field I have a list of usernames wrapped in span tags with a title attribute holding the username. I'm using the following code to set the value of the textfield. However it only takes one username instead of adding to it. I also don't know how to add a comma automatically when a username gets clicked.

Code:

$(document).ready(function() {
$('span.contact').click(function() {
var title = $(this).attr('title');

[code]....