Username1, Username2, Username3

$( "#message_recipient" ).autocomplete({
    source: function( request, response ) {
        if($('#message_recipient').val().indexOf(",") != -1){
            var pieces = $.trim($('#message_recipient').val()).split(/[s,]+/);
            var last_piece = $.trim(pieces[pieces.length-1]);
        } else {
            var last_piece = $.trim($('#message_recipient').val());
        }

        if(last_piece!=""){
            $.ajax({
                url: baseurl+"users/search_for_user",
                dataType: "json",
                type: "POST",
                data: {
                    user_name: last_piece
                },
                success: function( data ) {
                    response( $.map( data.result, function( item ) {
                        return {
                            label: item.user_name,
                            value: item.user_name
                        }
                    }));
                }
            });
        }
    },
    minLength: 2,
    select: function( event, ui ) {
        $('#message_recipient').val($('#message_recipient').val()+ui.item.label+", ");
    },
    open: function() {

    },
    close: function() {

    }
});