To allow your employees to manage their own calendar sync go to Settings > Absence settings > Calendar subscriptions
From here you will be able to switch on what you would like them to see
Once you have set these options your employee will see this option above their calendar
Depending on what permissions you have set they will now see various options
When they click on generate URL this will give them a unique URL to copy into their chosen calendar.
N.B Do not refresh this URL as it will stop the current URL from working.
If syncing with Google calendar click here
If syncing with Outlook 365 click here