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.
After Registration the user will land at the default ownCloud login page.
Depending on which options the administrator had set up for the server, you might need to provide your key storage first. This is done in the Security section of your Personal Settings:
When you select Local Storage there, the key which was created on the registration page will show up again and will be ready for use.
Logged in, you can upload encrypted files using the Upload menu item. All files uploaded will only be readable the owner of the folder and all users the owner shared this folder with.
You can only view files AFTER you registered. Anything what was uploaded before is not viewable with your private key without further actions by the sharer.
Settings / Key Management
E2eeShare settings are found in the Security section of the Personal Settings panel.
Depending on the server setup, there you can generate a new private key or download the current one.
The key management is the same as for non guest users. You may find instructions at E2EE - User Setup.
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.