I'm thinking about jumping on that sale Unity has on their Unity Plus subscription at https://store.unity.com/offer/unity-plus , but I wonder if upgrading my own licence will give me any grief when working together with friends who are using the free version. I'm sure there was something in a FAQ once about everyone on a project having to use the same version, but it doesn't seem to be in the FAQ now.

Do anyone know how that works?

The software works just fine.

From the way I understand the license:

If your group meets the requirements for needing the pro version or if you release the title using the pro build, then everyone on the team needs the Pro license.

If the group meets the standards for the free version, then as long as the final build is made with the free version and includes all the free-version elements like the Unity splash screen you are fine. If someone happens to have their own license they can do their work with their own pro licenses without an issue.