Indeed, as jmwking suggested, there are online collaboration features for PowerPoint slides already existing, but they may require that you use the Microsoft online services for that. Instructional page on how to use the features is located here:

https://support.microsoft.com/en-us/offi...be-89cf2892a34d

Of course, if some users fork off their own version and edit it locally, you still have the same problem, but at that point, it becomes their problem rather than yours.