When you configure hybrid mode, calendar sharing is automatically configured between on-premises and cloud users. You don't need to configure anything. However, in some cases, you may need to perform an IISreset.
On a recent project implementing implementing hybrid mode for an Exchange 2010 organization, we enabled hybrid mode and tested connectivity. Everything looked good for mail flow and mailbox moves. However, free/busy lookups failed from the cloud users to on-premises. When we tried to view the on-premises users' calendars the status was "No Information".
When you're not sure what's going on, it's always a good idea to use the Remote Connectivity Analyzer to test things out. There is an Office 365 tab which includes a Free/Busy test. When I ran this test, it was successful, as shown in the figure below. However it still wasn't working.
In this case, hybrid mode had been configured for multiple days. The free/busy lookups required an IISreset or a server reboot to fix.
This fix is certainly relevant for Exchange Server 2010. It may also be relevant for later versions of Exchange Server.