def add_user_to_group(self, username, group):
        """Add a user to an existing group.

        :param username: Username that will be added to specified group.
        :type username: str
        :param group: Group that the user will be added to.
        :type group: str

        :return: json response from Jira server for success or a value that evaluates as False in case of failure.
        :rtype: Union[bool,Dict[str,Any]]
        """
        url = self._options['server'] + '/rest/api/latest/group/user'
        x = {'groupname': group}
        y = {'name': username}

        payload = json.dumps(y)

        r = json_loads(self._session.post(url, params=x, data=payload))
        if 'name' not in r or r['name'] != group:
            return False
        else:
            return r