To sync payroll costs from Xero to Procore, you must use the project tracking category on the timesheets function in Xero. For information on tracking categories, see this article from Xero, note that Smoothlink supports timesheet payroll syncing, not employee groups.
In your organizations Settings > Payroll Settings there is an option to set the tracking category for timesheets:
Make sure to use the same tracking category that is used to track projects between Procore & Xero. Once this is set when creating timesheets you can set the project on the timesheet:
Then, once it is paid it will sync across to Procore as a payroll direct cost, e.g. :
Note that there must also be a payroll cost code set in the defaults section for it to sync across: