I've always used Easy PHP Calendar for my clients. http://easyphpcalendar.com/ It is very flexible and I believe it meets your requirements. Worth looking into, anyway. If you've found something else, I'd be interested in hearing what you came up with, as I'm always on the lookout for great scripts. :-)
All the best!
User Zick developed a calendar that integrates with CMSB a while ago. You can find more in a very lengthy post at http://www.interactivetools.com/forum/forum-posts.php?71673 and download a complete set of files from my CMSB Cookbook at http://www.thecmsbcookbook.com .
You could add the multiple users and access control through standard CMSB approaches like the membership plugin.
Hope that helps,
Take advantage of a free 3 month trial subscription, only for CMSB users, at: http://www.thecmsbcookbook.com/trial.php
Thank you for the information. The Easy PHP looks like it will fulfill my customer's requirements. Since we have no need for the membership plugin I think it is a better solution for this project. But, for the CMSB calendar, does anyone know a link to a working version so that I can see the finished product on a website?