CKShare provides a url that allows others to be invited. It is necessary for a potential participant to have access to this url (otherwise there is no way for them to accept the invitation). An easy solution is to send this url via the Messages application, but this is an extra step for the share owner. I have noticed that Apple's Passwords app somehow sends this url to the invited user within the Passwords app - and I wonder if this is possible with just public Apple apis, or if Apple uses some private api to achieve this.
Topic:
App & System Services
SubTopic:
iCloud & Data