I have a Web site (IIS, Windows Server 2008) where users are constantly adding information to their pages.
How can I know that there currently are users working on my site?
How can I update the site without interfering with the users's activities?
Open perfmon. Add Web Service -> Current Connections counter. Use other counters such as Current Anonymous Users as appropriate.
That depends entirely on what exactly you're updating and the nature of your website. You may not be able to update it without disrupting user sessions. Schedule maintenance.
Another good strategy is to have a duplicate web server. Have both web servers behind a load balancer. Take one web server out of the load balancer rotation. Wait for user connections to bleed off of it. Update it once no one is on it any more. Put it back on the load balancer. Repeat for other web server.
Well I can't speak to how you can update your site without interrupting your users sessions, however I can tell you that WireShark will definitely help you track down who/what/where users are on your site at any given moment.