// Provide a submit button.
      $form['submit'] = array(
        '#type' => 'submit',
        '#value' => 'Get Employee',
        '#weight' => '4',
         '#ajax' => array(
            'callback' => 'rolealter_form_ajax_submit',
            'wrapper' => 'project_node_form',
            'method' => 'replace',
            'effect' => 'fade',
          ),
      );

    $form['actions']['submit']['#submit'][] = '_custom_form_submit';
}

$query->join('users', 'u', 'n.entity_id = u.uid'); //JOIN node with users

        $query->groupBy('u.uid');//GROUP BY user ID

        $query->fields('u',array('uid','name'))//SELECT the fields from node
            ->condition('n.field_skillss_tid', $form_state['input']['field_skills']['und'],'IN');
        $result = $query->execute();
        $form['field_user_lists']['#access'] = TRUE;
        while($record = $result->fetchAssoc()) {
            print_r($record['name']);
        }
 }else{
     $output = 'Enter a value';
 }