def specified_users(users_arg, users):
    """Parse a comma-separated list of users
    
    Check that all specified users are valid, and return them as a list
    
    Parameters
    ----------
    users_arg : str
        Comma-separated list of users
    users : container
        Container of valid users
    
    Returns
    -------
    list
        List of specified users
    """
    if set(users_arg.split(',')) <= users:
        users = users_arg.split(',')
        return users
    else:
        raise Exception(
            'Some of the specified users have not yet been inducted into the policy. Use minduct -u to induct them before continuing.'
            )
