How to get WordPress Username in Array format

I want to create an Autocomplete function in WordPress. I want a search field from where Username can be searched. I am using following JQuery UI.

<label>Users</label>

<input type="text" name="user_name" id="user-name" />

<?php

$get_arr_user = array('John', 'Rogers', 'Paul', 'Amanda', 'Peter');

?>

<script>

jQuery(document).ready(function($) {                                
var availableTags = <?php echo json_encode($get_arr_user); ?>;
$( "#user-name" ).autocomplete({
source: availableTags
});
});

</script>

My problem is that I am not able to get the list of Username in this format – array('John', 'Rogers', 'Paul', 'Amanda', 'Peter');

Plz help me in getting the list of Username in array(). Thanks..