Update People Field PowerShell

I'm trying to update a People field that contains multiple users. 

I found this example:  http://www.sharepointdiary.com/2016/10/add-remove-user-in-people-picker-field-value-using-powershell.html

I keep getting the following error:

Exception calling "Update" with "0" argument(s): "Invalid look-up value.

I've not found anything specific.  I know it is tied to the people field and choosing multiple people in it as I have no other fields in my list items. 

David Jenkins