How can I manage iCloud storage for a business of over 100 people?

iCloud is aimed at individuals or families. It is not aimed at businesses and does not come with the level of support a business would expect. The only way for 100 people to use it is for each of them to use the same ID and password, and this might be possible depending on exactly what you are expecting the service to do, but could also cause problems. It's also very possibly against Apple's Terms of Use*. Remember also that iCloud is a syncing service designed to keep all your devices in sync. It's not a separate storage service. The contents of iCloud drive would be mirrored on each signed-in computer and anyone signed in can modify or delete a file, which would effect every other copy. There are no permissions levels.

I would think you would be better off investigating a dedicated business solution such as Amazon S3.

*Indeed they say: ' You further acknowledge and agree that the Service is designed and intended for personal use on an individual basis and you should not share your Account and/or password details with another individual."

https://www.apple.com/legal/internet-services/icloud/en/terms.html