Users in remote locations can sync calendars with each other through Google Calendar without having to share a single Google Calendar account. Each of the users can sync with their personal Google Calendar account, and then use Google Calendar's sharing features to share calendars with each other.
For example, Fred syncs to his personal Google Calendar account, and Mary syncs to her personal Google Calendar account. Then Fred shares a calendar with Mary using the Google Calendar sharing interface, so that the calendar shows up on Mary's Google Calendar. Once there, it will sync to Mary's Mac. And Mary can make changes to that calendar on her Mac and it will sync back to Fred's Mac through Google Calendar.
In this example, we start out with Fred and Mary each syncing BusyCal with their personal Google Calendar accounts, but they are not yet sharing calendars with each other.
Fred can share his calendar on Google Calendar with Mary by logging into Google Calendar on the web and clicking Settings > Calendars > Share this calendar.
In the next dialog, Fred enters Mary's Google Calendar account ID in the Person field, and sets the Permission Settings (Make changes to events, See all event details, or See only free/busy time) for the calendar. In effect, Fred is Publishing his calendar to Mary's Google Calendar account.
Once Fred has shared his calendar with Mary, it will show up on Mary's Google Calendar, and from there it will automatically sync to BusyCal on Mary's Mac where she can subscribe to it.
Now Fred and Mary can both make changes to the Fred calendar in BusyCal and the changes will sync back and forth through Google. This process can be repeated with other calendars that Fred or Mary wish to share with each other or with other Google Calendar users.