Working with Person Group column is a common task in SharePoint development. Following illustrates how to update a single and multiple values to SharePoint person group column.
Insert / Update Single Value SharePoint Person Group Column (People Field)
Tip: For single value you should refer the person or group by ID
Insert / Update Multi Value SharePoint Person Group Column (People Field)
Tip: For multiple users you need to refer it by login name
*listitem this is the list item you need to manipulate. If you are inserting a item this will be created using ListItemCreationInfo and if you are updating a item this will be a reference to the item you seek