code: def invite(self, group_name, user):
          
          group_id = self.get_group_id(group_name)
          self.params.update({
              'channel': group_id,
              'user':    user,
              })
          return FromUrl('https://slack.com/api/groups.invite', self._requests)(data=self.params)
docs: https://api.slack.com/methods/groups.invite