BusyCal will allow you to sync calendars between multiple user accounts on a single Mac. Syncing calendars on a shared computer is the same as syncing calendars on a LAN — each user account on your Mac will appear as if it is a separate computer on your LAN.
To sync calendars between different user accounts, you must enable fast user switching and periodically login to all user accounts so that BusyCal is running simultaneously on all user accounts so it can sync calendars between them. If only one account is logged in, any calendar edits made on that account will be treated as offline edits and will be synchronized the next time all accounts are logged in.