I dont have an own server yet, nor know any other nice private servers.
I only know having shared folders among users of the same server. Federation would probably be that, controlled by permissions, for multiple servers.
Haha wanna cry a bit here. My Uni loves Microsoft BS, they actually have a License for all of use for all products, as well as chain us to stupid DRM software.
Now I asked them if they could offer OUR timetable not only as MS Exchange but plain old CalDAV, as Exchange is not supported by ANY FOSS software natively. Long story, end “use this weird app, it works trust me bro” and “no we wont support it”.