code: def roles_add(user, role):
      
      user, role = _datastore._prepare_role_modify_args(user, role)
      if user is None:
          raise click.UsageError('Cannot find user.')
      if role is None:
          raise click.UsageError('Cannot find role.')
      if _datastore.add_role_to_user(user, role):
          click.secho('Role "{0}" added to user "{1}" '
                      'successfully.'.format(role, user), fg='green')
      else:
          raise click.UsageError('Cannot add role to user.')
docs: Add user to role.