I downloaded and installed XCODE beta 3 and this resolved this issue. You can consider this closed.

I should have noted: I can add a defaultSize to the new windowGroup but if the current window is full-screen, this default is ignored. If the window is not full-screen, then the default size is STILL ignored, but the two windows appear stacked with an offset on top of each other. So this is closer to what I am looking to accomplish, but I need to have it work for both FULL-SCREEN and WINDOWED configurations.

I upgraded from macOS beta 3 to beta 4, then installed Xcode beta 4. As soon as the simulator started, it popped up a notification that Apple Intelligence was ready to use. The FoundationModel code now works in my app.

Filed FB19122234!

Ziqiao: Thank you for getting back to me on this issue. I have moved my code to use ShareLink; I am using SwiftUI and this looks like the best match. I was hoping to be able to use the CKShare to track who was invited to the Share. But, in my experiments, I am not seeing any invitations reflected into the CKShare's participants before they accept the invitation. Is there any way to use ShareLink to populate the CKShare's participants when they are 'invited' but not yet have accepted the invitation? [I am still working on getting 'accepting' working in my test environment, so I might still have more questions once I can see that part of the workflow working]. Thanks in advance, Charlie

Looks like doing a schema reset in CloudKit console cleared this issue. Perhaps the 100 user limit in a zone also applies to 100 invitations to the same user ;-). Closing this.