True if the message was sent on a group or megagroup.

def is_group(self):
        """True if the message was sent on a group or megagroup."""
        if self._broadcast is None and self.chat:
            self._broadcast = getattr(self.chat, 'broadcast', None)

        return (
            isinstance(self._chat_peer, (types.PeerChat, types.PeerChannel))
            and not self._broadcast
        )