In single primary the other nodes are put in a read-only mode so they can not accept writes. So stale reads can not happen.

To expand on your question a little - the MySQL router currently works better for single primary (it supports multi-primary but does not prevent stale reads). If you go third party, ProxySQL also supports Group Replication natively with multi-primary:

http://lefred.be/content/mysql-group-replication-native-supp...

I was chatting to the author about this at FOSDEM - since it is possible to keep track of nodes state by following GTIDs executed in the binary log stream. I believe this is how ProxySQL is doing it, but it's possible the final implementation differs :-)

When you read from non-primary nodes in Group Replication you get stale reads. There is no protection in place.