Cannot add members from my family group to new PC

Hello - it gives me slight relief that it is not only me experiencing this.  But! I have discovered a solution that is similar to what andadm​ states above:

• Settings > Accounts > Other Users – click Add Account

• This will pop up:

Click “I don’t have this person’s sign-in information”

• This will then pop up:

Click “Add a user without a Microsoft account”

• The Create a user for this PC wizard will then pop up:

Proceed to create a local account for each of your family. 

Note: Before Microsoft Family existed, I remembered that this was how I made different local accounts for different users (re: my family) in my PC about 5 years ago. So, it’s going back to old-school ways of doing things. 

• Log in to the account you created. Open Settings and in your Home page, there should be an option to sign in your family member’s MS account:

You should be able to sign in without a hitch. 

I know that this is a bit of a roundabout way to handle this bug, but it is a solution that worked for me.  Takes more work (the “allow sign-in” in the Accounts > Family was such a time saver), but this will do for now while MS developers (hopefully) fix the bug.