SharePoint User/Group Columns

SharePoint User/Group Columns are just like normal SharePoint Lookups except that the target list is the User Information List in the current site.

When attempting to set a User/Group value to a list item that user must exist in the User Information List so that it has an ID number than can be referenced.

You cannot simply sync users into this list it must be done via an elevated API call so that its created correctly in the SharePoint Database. Therefore you should ensure that your users are added as individual user accounts under the SharePoint site permissions.

Using LOOKUPA/B Function

You can use the LOOKUPA/B calculated column function to get the ID of a User from the User Information List.

LOOKUPA("ID", "User Information List", WHEN("Name", value))

To convert this into the SharePoint Lookup value syntax you could use an expression like this.

IF(ISNULL(LOOKUPA("ID", "User Information List", WHEN("Name", value))), NULL(), FORMAT("{0};#{1}", LOOKUPA("ID", "User Information List", WHEN("Name", value)), value))