There is no 3rd person specifically discussed or included in ' avec laquelle nous étions parties ' - only Amélie and Joséphine. 

There is a prior reference to 'j'ai également fais des rencontres extraordinaires ...', but the discussion  soon continues specifically and uniquely about Joséphine (and Amélie). 

The French grammar "nous étions parties" is correct here. The French expression does not translate directly, word for word, to a standard English 'turn of phrase', but that doesn't make it incorrect French. 

Thanks Maarten. I must say, l don't yet get  it ... yet ... 😀 if 'avec laquelle' refers to Joséphine then l read, 'nous'  as Amélie and someone else .... unless there is a French royal 'nous'. 

Anyways, if this is the correct answer then l will figure it out at some point. I appreciate your statement about not getting stuck with literal translations. 

I see your problem with the sentence:

Je pense notamment à Joséphine, que j'avais rencontrée au détour d'un sentier alors que je visitais la forêt millénaire de Brotonne, et avec laquelle nous étions parties à la recherche du célèbre chêne Cuve. -- I am thinking specifically of Joséphine, whom I had met along a path while visiting the ancient forest of Brotonne, and with whom we had set out in search of the famous Cuve oak

You see Valérie talking and expect that "we" refers to Valérie and somebody else (presumed, by default, to be male) who left together with Joséphine. Your question is a question of context only, not of grammar. And, as Maarten points out, there never is mention of a 3rd person. Hence, the "we" refers to the two women.

The point, Chris, is that you can't do that in English. If you use "we" there, then it would definitely mean that someone else was involved, so it has to be translated as "with whom I had ..."

I had no idea that this was possible in French, and almost didn't believe it, but I was able to find an example in a book where "nous" is indeed translated as "I".

I would be interested to see some more examples like this - does it only apply to this specific structure with "avec lequel"?

A similar example (from someone's doctoral thesis):

(11) avec des amies, nous sommes allees voir [...]

Predicates sometimes repeat the content of the phrase with “avec.” In Example (11), “nous” means “I” and “friends” which are already mentioned (”avec des amies”) . 

So interesting...and helpful.... after reading  the examples you gave Alan, l followed up on my previous hunch (prior post) that there might be a "royal we" in French, in which 'we' refers to one person, as we hear King Charles lll often do. My search immediately surfaced some interesting articles entitled, "le nous de majesté" l guess doctoral theses also require such formal language.  Fascinating! Thanks to you all, this is starting to make sense...l actually recall having had that hunch during the dictée, so there must be something in the dictée that points to that. I will go digging 😀

But this is not the "royal we", and the thesis is not using formal language - it's giving an example of normal French. What's happening is that the speaker is combining themselves with the person/people marked with "avec" to form a group that requires "nous". We never do that in English so it's a bit confusing.

I had a chat with a French friend of mine in France and she did not bat an eyelid. She immediately saw Josephine and Amélie being folded into the 'nous', alone. The phrase construction is very familiar her, automatic. She said that when the 'nous' is used like that, the best thing is to just decline using those explicitly named.

This dictée has proven to be a gold mine for me if only for this one phrase construction, which l was totally oblivious to. It is starting to feel more familiar now.

Yes, this sentence illustrates a difference in how English and French speakers interpret and think about "we". I think this should be worth a hint.