When sending timesheets from WorkGuru to Xero, the first step is to match the WorkGuru timesheet users to Xero's payroll employees in the payrun. By default the users are matched to the employees by name and email address. If these are different between the two, you can force a match by linking directly to the employee UID.
In Xero, go to Payroll -> Employees and select the employee you need to match. Copy the employee UID string from the URL in their details page.
Open the user's detail page in WorkGuru and paste the employee ID in the Payroll System ID field and click Save.