Any calendar you've shared with a given email connected to your Reclaim account will show in the list of calendars that you can pick from when setting up the sync policy. To sync a shared calendar, you follow the exact same steps as you would for setting up any other sync policy.
Navigate to the Calendar Sync page in Reclaim
Click the + New Sync button in the top right of the page.
Under the 'Source' section, click the downward arrow next to 'Select Account' and choose the desired Google email address to see a list of your shared calendars associated with your Google account.
Select the shared calendar you want to sync.
Under the 'Destination' section, click the downward arrow next to 'Select Account' and choose the desired Google email address to show shared calendars associated with your Google account that you want to make the destination for the sync'd events.
Click Next: Sync Details.
Customize your syncing details.
Click Start Syncing to complete setup.
Learn more about the differences between primary and shared calendars here.