@grishka Looks interesting, I think writing a FEP is a good idea. The process is not dead yet
I want to implement private groups too but it is not clear which approach is better. We discussed private groups with @nutomic and he said that FEP-1b12 can also support private groups. There is an RFC, but AFAIK it has not been implemented yet.
And here's the documentation for @mikedev 's Conversation Containers: https://fediversity.site/help/develop/en/Containers