This is the most straightforward method, in a custom module:

  $role = user_role_load_by_name('role-name');
  user_save($account, array('roles' => $account->roles + array($role->rid => $role->name)));
}

This is the most straightforward method, in a custom module.

function MY_MODULE_user_insert($account, $data) {
  $role = user_role_load_by_name('role-name');
  user_save($account, array('roles' => $account->roles + array($role->rid => $role->name)));
}