In my current work, and at various previous companies, I've done a lot of coordination with external groups. We generally use email for this: it's very standard and works well enough.

One place where it doesn't work very well, though, is making it possible for others working on a project to read and find relevant threads. For example, if I write to our partner I can CC coworkers who are currently on the project, but what if my boss is interested? What about other people who might join the project later? What if I want to share a link to the conversation with a co-worker?

My preferred solution here is email lists. For example, if I were at ExampleCorp working on Project Cabbage with OtherCorp, I would create a cabbage-other@example.com list and CC it on all my Cabbage-related messages to OtherCorp. If the Cabbage-Other collaboration isn't sensitive I might have the list set so anyone at ExampleCorp can join, or I might open it only to specific people at ExampleCorp. (This is overkill if Cabbage is a small project; then I'd just use cabbage-collaborations@ or some other single address for everything external Cabbage-related.)

This also makes CCing someone a much clearer signal: without this, it's hard to tell the difference between being CC'd on a message "so you can read it if you want to" vs "because you probably should be following this thread." But if I receive a message because I'm on the cabbage-other@ list that's not something I necessarily need to read, while if I'm explicitly listed in the CC line I'll pay more attention.

I can also tell my mail client about this: I'll subscribe to all of these lists but set them to skip my inbox if I'm not specifically mentioned. [1] Then they show up in my Gmail search, and if I do later want to reply to one (ex: we're moving work around; someone's out) it's easy.

[1] In Gmail: on a message from the list click "filter messages like this", verify it has a "has the words" of something like list:(...), expand that to append -to:me -cc:me, click "create filter, check "skip the inbox".