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 at the bottom of the page.
Under the 'Sync from' section, click the downward arrow next to the box showing only your email address to see a list of your shared calendars associated with your Google account.
Select the shared calendar you want to sync.
Click Next: Preferences.
Customize your syncing details.
Click Start Syncing to complete setup.
Learn more about the differences between primary and shared calendars here.