Users will receive a unique ID, if a user enters another user's ID they will go to a view where both have access to the information, being able to change, add, delete...
(Paired, available on App Store) Public container is not secure, private with ckshare doesn't seem to work for what I would like, plus the content is very confusing
I need something that uses native Apple technologies to build this system.
The screenshots you provided look similar to the invitation UI of the CloudKit sharing flow. If you have already looked in to CloudKit sharing (CKShare) and concluded that it doesn't fit your use case, I don't see anything better, and would suggest that you file a feedback report with your reasoning. Otherwise, the Sharing Core Data objects between iCloud users sample may give you a quick start.
Best,
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Ziqiao Chen
Worldwide Developer Relations.