I mean the encryption logic is interoperable anyways all apps can decrypt DMs. The keys can then be shared around as gift wraps, whatever client understands the protocol for sharing can let you access the private events, I created a nip and a PoC sometime ago I have to spend some time to get it production ready

GitHub

NIP-FR: Add Friends-Only Notes by abh3po · Pull Request #2207 · nostr-protocol/nips

A mechanism to share encrypted notes with different sets of people