Have you tried to see if a user can logon to a different machine to determine if the desktop sharing ability is available?
If a user currently cannot initiate a desktop / application sharing session but does have the ability after logon to a different machine that did not have the issue before. Chances are the machine has a local policy restricting the functionality.
If the user continues to experience the issue regardless of machine, then you have a setting in the OCS pool or domain. Unfortunately, this is the extent of how far I can go with OCS.
