I wanted to update person or group field with single or multiple set of users.
Following approach worked for me.
To update it with single user account, retrieve the value of the corresponding User Name property for the user account you would want to update from the list item, example user1@domain.COM.
To update multiple set of users for person field. Gather the User Name property value from list and concatenate the values using semi colon to look like this example user1@domain.COM;user2@domain.COM;user3@domain.COM;user4@domain.COM.
Trim for any spaces in between and trim end for semicolon.
Finally, use Update List Item action in Nintex workflow to make update to SharePoint list.
Note: These are not the email ids.