Here are the steps to setting up a group account with with managed sync licenses:
1. Follow Authorizing Access for Managed Syncs. Do this for each of the tenants that host calendars you want to sync.
2. Follow Create a Group Account, being sure to purchase sync (not user) licenses.
3. Follow the steps in Connect authorized domains to group account. Do this for each of the tenants that host calendars you want to sync.
4. Go to Managing Group Licenses and follow the steps to “Assign a sync license / create a managed sync connection.”