BusyContacts enables you to share address books with other BusyContacts users.

Address book sharing is supported on the following services:

• Exchange — Exchange supports address book sharing with BusyContacts. You can run your own Exchange server in-house or use a hosted service like Office 365.
• Fruux — Fruux is a CalDAV and CardDAV hosting service that provides native support for address book sharing with BusyContacts.
• iCloud — Address book sharing is not supported by iCloud. Therefore, if you want to share address books on iCloud, the only way to do it is by syncing with multiple iCloud accounts.
• Google — Address book sharing is not supported by Google. Therefore, if you want to share address books on Google, the only way to do it is by syncing with multiple Google accounts.
• Kerio — Kerio supports address book sharing with BusyContacts when syncing via CardDAV or Exchange/EWS protocol..
• Zimbra — Zimbra supports address book sharing with BusyContacts when syncing via the Exchange/EWS protocol.
• LAN — you can share address books over the LAN peer-to-peer with other BusyContacts users. The main limitation to this approach is that you can only share local address books, which cannot be synced with an iPhone.

For more info, see Selecting a Cloud Syncing Service.