You'll have to send their payroll payment outside QuickBooks Online (QBO).
You can prepare and run your payroll in QBO and submit your FPS (Full Payment Submission) to HMRC. At this time, you can only email the payslips to employees if you're using QuickBooks Online Standard and PaySuite.
I can see that it would be a great addition to the current features that we have. I'll take note of this and let our Product Development team know that this is something that you need.