There's an Open Source caldav and carddav plugin for Outlook:
Note: the funambol client doesn't work with Office 64bit and Office 2013. Use the Bynari client for those versions.
Outlook can synchronize using the free Funambol Windows client. You can get this client from the Funambol website (Be careful, don't click on the "latest version" link because that links to the Funambol server): http://sourceforge.net/projects/funambol/files/windows-sync-client/v10/
Follow the installation instructions and make sure you use the following settings:
- Don't use encryption. You can use an SSL connection if you want to be secure but Group-Office does not support the DES encryption from the funambol client.
- Make sure the SyncML URL is set to http://your.url.to.groupoffice/sync. If this doesn't work then an alias is missing in the webserver configuration. You can try to connect to http://your.url.to.groupoffice/modules/syncml/index.php in that case.
The default settings should work but if something goes wrong check the advanced settings of each datastore: