Within the scope of an application’s cloud container, each user has a unique, application specific iCloud user ID and a user info record.

Swift - CloudKit - Unique User ID

Within the scope of an application’s cloud container, each user has a unique, application 
specific iCloud user ID and a user info record where the user ID is used as the record ID 
for the user’s info record.
The record ID of the current user’s info record can be obtained via a call to the 
fetchUserRecordID(completionHandler:) method of the container instance. Once the record 
ID has been obtained, this can be used to fetch the user’s record from the cloud database:

container.fetchUserRecordID(completionHandler: {recordID,    
        error in
            if let err = error {
                // Failed to get record ID
            } else {
		   // Success – fetch the user’s record here
	      }
	      
The record is of type CKRecordTypeUserRecord and is initially empty. Once fetched, it can be 
used to store data in the same way as any other CloudKit record.
CloudKit can also be used to perform user discovery. This allows the application to obtain an 
array of the users in the current user’s address book who have also used the app. In order for 
the user’s information to be provided, the user must have run the app and opted in to provide 
the information. User discovery is performed via a call to the 
discoverAllIdentities(completionHandler:) method of the container instance.

The discovered data is provided in the form of an array of CKApplicationUserInfo objects 
which contain the user’s iCloud ID, first name and last name. The following code fragment, 
for example, performs a user discovery operation and outputs to the console the first and 
last names of any users that meet the requirements for discoverability:

container.discoverAllIdentities(completionHandler: (
     {users, error in

        if let err = error {
            print("discovery failed %@", 
                   err.localizedDescription)
        } else {

            for userInfo in user {
                let userRecordID = userInfo.userRecordID
                print("First Name = %@", userInfo.firstName)
                print("Last Name = %@", userInfo.lastName)
            }
        }
    })