https://api.slack.com/methods/groups.mark

def mark(self, group_name, ts):
        """ https://api.slack.com/methods/groups.mark
        """
        group_id = self.get_group_id(group_name)
        self.params.update({
            'channel': group_id,
            'ts':      ts,
            })
        return FromUrl('https://slack.com/api/groups.mark', self._requests)(data=self.params).post()