Function to get the name in database table from the comma separated string

Not tested but this should work:

add_action('manage_pages_custom_column', 'display_page_custom_cols', 10, 2);
function display_page_custom_cols($col_name, $post_id) {
  global $wpdb;
  $user_group = $wpdb->get_results("SELECT * From custom_user_group",OBJECT_K); 
  if ('user_group' == $col_name) {
    $pages = explode(',',get_post($post_id));
    $output = array();
    foreach ($pages as $page ) {
        $output[] = $user_group[$page]->GroupName;
    }
    echo implode(',',$output);
  }
}