Yes, that is the correct usage for user accounts. When you create the XML import file, you can also specify which roles the user is enrolled as -- eg. reader, presenter, etc. The Conference Director can use the Notify Users function to notify different groups of users via email, and the Conference Manager can also bulk-email selections from a particular role or all enrolled users (even by role) at once. If you have any specific questions on how to do this, please ask. I'd also be interested to hear how moving 30 years of conferences to OCS goes -- that sounds like a very ambitious project!