Hi Michael, I think I may have figure the reason, just not the solution. I get the error on the "Dropdown Box" that says...
Warning: The columns produced by this rule are all nested tables and/or records, however the property expects at least some columns of simple values (such as text, or numbers)
and from what I remember, this is because the SP List type of "People and Groups" is a group of attributes such as "Name", "Email", etc so the "Dropdown {DD} Box" can handle that. Somewhere deep in the back of my brain, I remember something about having to create a collection to display all the attributes and then use the collection to populate the DD.
I guess I have some Googling to do 🙂