def validate_group(group_name):
    """Check that a group of users is not named ``free`` or ``shared``
    
    Parameters
    ----------
    group_name : str
        Group name to be validated
    """
    if group_name == 'free':
        raise Exception(
            'Can\'t free memory from the "free" group... it\'s already free.')
    elif group_name == 'shared':
        raise Exception('"shared" can\'t be a group name')
