Thanks Justin. Got it to work! I shut down my iMac, rather than restarting, and now - in the TV app on my iMac - I see the members of my family, as well as the movies they purchased.
One thing I noticed is that the menu in the left column of the TV app on the iMac/Big Sur is different from the iPad and Apple TV: In the TV app on my iMac, I have to click on the (arrow behind) the word "Library" at the top of the left column, to see the family members. Then selecting a family member allows me to see the movies purchased by each family member.
Justin, I really appreciate your fast response and helping me out.
Best wishes!