theoretically speaking..

implement a default encryption on all a users posts/comments/events, distribute decryption keys to any user the same way you'd accept a follow request on a centralized platform.

i'm not well versed in this area, but maybe something like what nostr:npub1zuuajd7u3sx8xu92yav9jwxpr839cs0kc3q6t56vd5u9q033xmhsk6c2uc is implementing in #whitenoise for encrypted group chats.