You can install bitmessage authorization listener to as many computer you want to avoid the single point of failure.
The authorization providers has 2 purpose:
If you don't need this features you can create a zeronet-only authorization provider by publishing the private key, but it's not recommended.
No Bitmessage support built-in zeronet, but you can register by downloading the client and sending "add:#$auth_address:#$username" message to BM-2cUhFY6Ay2LSZZTJ1por17uRWa2oXGQKuK
The source code is here: https://github.com/HelloZeroNet/ZeroID
Basically you have to return a cryptography signed useraddress#web/username
string to user.
He saves the sign to data/users.json
and then using it he will be able to publish content to sites that accepts his authentication provider.