If a Nostr relay requires AUTH—as based on a paid subscription—it would be one way a pubkey/npub could be associated with a group ID (the only metadata available, afaik). The other would be based on an IP, but that is similar to Signal et al (and mitigated with VPNs and etc). E-cash 'blind signatures' seem to be the best solution for relay 'subscriptions' without knowledge of a pubkey. It was designed for this scenario, even if from 1981.