Photos app on iMac won't allow addition of new people to Shared Album invitees list

On my iMac, I set up a new Shared Album in Photos without inviting anyone to the Shared Album. Later, I wanted to add folks, so I clicked on the "person" icon, the image on top is what I saw. The bottom image on the right is what I saw when I tried exactly the same thing on my MacBook.

As one can see, on my iMac the "Invite People" box simply is missing!

Both iMac and MacBook are running 10.15.7.

Any thoughts as to why this might be happening? I couldn't find any setting that might speak to this.