Return the list of users in the specified group.

    :arg config: a dict containing the fedmsg config
    :arg fas: a fedora.client.fas2.AccountSystem object instanciated and loged
        into FAS.
    :arg groupname: the name of the group for which we want to retrieve the
        members.
    :return: a list of FAS user members of the specified group.

def get_user_of_group(config, fas, groupname):
    ''' Return the list of users in the specified group.

    :arg config: a dict containing the fedmsg config
    :arg fas: a fedora.client.fas2.AccountSystem object instanciated and loged
        into FAS.
    :arg groupname: the name of the group for which we want to retrieve the
        members.
    :return: a list of FAS user members of the specified group.
    '''

    if not _cache.is_configured:
        _cache.configure(**config['fmn.rules.cache'])

    key = cache_key_generator(get_user_of_group, groupname)
    def creator():
        if not fas:
            return set()
        return set([u.username for u in fas.group_members(groupname)])
    return _cache.get_or_create(key, creator)