Nostr identities are entirely self generated, and there's no need for a traditional registration with each community. A single invite link could theoretically convey all the information required to join a community. Exact implementation will depend on the relay that hosts the community and the software they use to do so, but there's no explicit need to make users register in a traditional sense, just join with the npub identity they created themselves. Some may make further requirements to curtail spam and other low quality content, but that becomes a decision for each individual community as best fits their needs.