Navigate to your Staff directory via the menu on the left.
Select Ex-staff to bring up a list of all past employees.
NB: Ex-staff can only be viewed by admin users.
Click on the cog icon to the right of the staff member's name and select Reinstate staff...
This will prompt a warning to appear on your screen to ensure that you want to reinstate this staff member.
Once you have selected Yes, the staff member will reappear in the staff list.