/*profile_2_type is id for profile 2 module and it's not working for me*/
$form['profile_2_type']['field_test_auto_cmplete']['#autocomplete_path'] = 'user/autocomplete';

/*Here is the code which is working for me for node fields */
$form['title']['#autocomplete_path'] = 'user/autocomplete';

$form['profile_2_type']['field_test_auto_cmplete']['und'][0]['value']['#autocomplete_path'] = 'user/autocomplete';

function  ajaxengine_form_cruiseline_reservation_node_form_alter(&$form, $form_state, $form_id) {

 $form['#after_build'][] = 'ajaxengine_after_build';