Syncing with Apple Contacts
You can sync BusyContacts with Apple Contacts on your Mac.
BusyContacts can sync with the built-in Contacts app on macOS through a cloud-based service (iCloud, Exchange, Fruux, Google, or a CardDAV server). If macOS Contacts is already syncing with a cloud-based service, it should automatically sync with BusyContacts once you've configured BusyContacts to sync with the same account.
BusyContacts will not sync local "On My Mac" contacts with macOS Contacts. The Setup Assistant will allow you to import local contacts from macOS Contacts, if they exist, but this is a one-time operation and the local contacts will not remain in sync between BusyContacts and macOS Contacts.