Answer
To add employees into a pay run they're missing from, go to the Pays tab and click on Create Pays Wizard for the specific pay run.
Then, select the employee(s) you want to add and click Create. If you tick the Create Full Pay? option at the bottom of the Create Pays Wizard screen, the program will automatically use the hours and pay rate set in Employees >> Pay Settings >> Pay Rates to pre-fill the pay details for you. Don???t worry??? you can still edit these pays as usual using the Edit Pays button on the main Pays tab.
If you've recently added a new employee but can???t find them in the Pays tab or through the Create Pays Wizard, it???s likely due to an incorrect Start Date set for the employee. To resolve this, go to Employees >> Details >> Employment Dates, update their Start Date to the correct date, then return to the Pays tab. The employee should now appear in the list for that pay run.