System administrator permission for 10to8 account and organisation calendar provider.
Make sure the Staff/Consultant email address on "Set Up" > "Staff" page matches with email on your third party calendar provider (in case of multiple email addresses, 10to8 will use first created email address).
Install the Cronofy application in G-suite
More details can be found here.
Office 365 and Exchange
Create Service account with ApplicationImpersonation role
Detailed steps can be found here.
Setting up Enterprise Connect calendar sync
Setting up your calendar sync can be useful in many ways, and it is easy. To get started go to "Set Up" > "Calendar Sync".
Select Link all calendars centrally option and click on Set Calendar Sync Authentication Mode:
Click on Connect all staff records to external calendars to authenticate all bookable staffs centrally.
Choose Office 365, Exchange or Google calendar, provide required details for authentication and all bookable staff’s calendars would be linked upon successful authentication.