@Niel Liesmons and I (and others) talked about this idea at NostRiga. Proof of membership. There is a public member list (in this case that's the relay-based-community list) and you want to prove your npub is part of that list without revealing to the verifier which npub u are. There is a way to do it by using zero-knowledge sets, however my understanding is limited. It should be similar to (and simpler than?) the work @waxwing is doing (tldr; prove that I own UTXOs that exceed the value of X sats, without revealing with ones)