You still need to first enable logging at the global level, but using the option only for selected users and not all users. But there are a few more steps (like deploying an archiving server first).
Start off by reading through the OCS 2007 Archiving Server Deployment Guide.
Once you have deployed an Archiving server and associated it with the existing Front-end server(s), then you can go about enabling it.
Then follow the steps on Page 11 under the section "To archive traffic for selected users" which will walk you through globally enabling archiving for internal and/or federated conversations by selecting the option(s) Archive according to user settings. At this point you can now select specific users in the management console and enable archiving for them only.
