I work on one of the third party plugins enabling real-time collaboration in Obsidian called Relay [0].

We have a novel architecture where you can optionally register a self-hosted relay server with our control plane for complete privacy for all of your docs and attachments.

We know that people typically prefer to have a unified vault, so you can share individual folders with different groups of people within your vault.

Relay is free for markdown docs up to 3 users, and then we have a hobby plan which includes attachment storage (especially popular with D&D and TTRPG players), as well as per-seat plans for businesses and universities. There are a couple of cloud-only alternatives like peerdraft and screen garden as well.

[0] https://relay.md