In order to register with a guest account you need to set a password on the provided registration page. The link to that page (e.g.) is sent by email after having shared a folder with a guest for the first time. .
For Registration you just have to enter and confirm the password. A private key will automatically be generated on the browser and the public key/certificate submitted. You can also backup your private key here.
The key management is the same as for non guest users. You may find instructions at E2EE - User SetupDocumentation.
If you see more keys than you added by yourself, or your current key does not match any more, notify the administrator immediately as this is a sign of an incursion attempt.