The Communicator Team has blogged how to use the conference resource in OCS 2007 and there some examples how to start a conference from Outlook Client as well.

Conferencing was a large investment for OCS and OC in the 2007 release. We define conferencing as a conversation having three or more participants in it. This can be IM, Audio, Audio/Video, data (Live Meeting), or any combination of those items. For Communicator, we worked hard to keep the fact that it is a “conference” fairly transparent in our user experience. In this article I will explain how easy it is to take advantage of conferencing assuming that you have a conference supported version of OC/OCS 2007.

Read more at source: http://communicatorteam.com/archive/2008/06/06/296.aspx