iCloud Drive isn’t quite like Google Drive or Dropbox. As with most Apple digital and cloud services, it’s tied to a single identity without much in the way of sharing. iCloud Drive's sharing features seem a bit tacked on.
Macworld reader Howard writes in asking about an aspect of this. “I was hoping also to have my wife’s iCloud drive show up on my Mac’s Finder the way I do with Dropbox. I haven’t been able to get this to work.”
It’s not you, Howard. You can only mount a given iCloud Drive associated with an iCloud account on an account in macOS logged into that same iCloud account. With some previous Apple cloud storage systems, you could use the credentials (user name and password) to mount a drive without having your system logged into the same account.