@malte Easiest (imho) would be a shared private GitHub repo, and one of the aforementioned plugins, with obsidian. This would be (afaik) free solution, and the issues can be solved by documentation. Github is well documented with text and video. It will not be hard to find outside guidance as well.

Easy to trail-run:
- subscribe to GitHub, create a private (empty!) repo
- follow the plugin docs you choose, the gitless one is simplest, but research ymmv
- test alone, then add one person, etc

@malte A lot will depend onthe skill level of the people involved. Also, how confidential is the data? Can it be on a server on the net or must it be local only?
Two good contenders:
- https://github.com/silvanocerza/github-gitless-sync
- https://github.com/Vinzent03/obsidian-git
Both have cooperation, conflict resolution, and work with Obsidian. They use GitHub as server, & can be public or private. Tech level medium. As user friendly as Obsidian.

@malte Yes, it takes care of the plumbing, if I remember correctly it doesn’t have much of a frontend

@malte There is a git plugin for obsidian, so it basically already works. What you would need is a more expansive fronten for git conflicts. It shouldn’t be too hard to either use a regular git tool (there are plenty), or expand/ fork the git plugin.
All the separate parts exist and are mature. The only thing really missing is a dedicated plugin. Might not be to expensive to have somebody code it for you.