Maybe one could modify an existing server based file sharing app and add a public/private key mechanism that only people who have shared their keys with the server will get to keep their files on the server, if they key doesn’t match what the client signed with and was pre-registered it gets deleted instantly. That would prevent CSAM etc from a hostile intrusion. Of course it requires vigilance of the group. I envision this working for small groups who have email or Signal contacts with each other. It needs that “it takes a village to manage a friends share” mentality.
