You can set up alerts (as emails) for specific item. Would that work for you? Another option is to create a view that would specifically show those events user has subscribed to and have outlook recieve RSS feed for this view. You can read more about Track updates on SharePoint site.
Excellent solution for you would be if you could "connect to outlook" but don't connect to all items, but just the view. OOTB however SharePoint doesn't offer such a feature. There is no ready solution, but there has been some effort to have it somewhat working... involves hacking/coding: