I'd need to enable a 3rd party hosted service from within Active Directory or Exchange server. Basically, IT admin can select a list of users from AD or Exchange server and say eanble "AAA" service for these users. The list of users with the information are sent over to the "AAA" service through web service and the accounts are then created for "AAA" service for these users.

Thanks in advance

I know that .NET has classes specific to manipulating Active Directory, I don't know if the same exists for Exchange Server.

The two main classes for Active Directory manipulation is DirectoryEntry and DirectorySearcher which are both part of the System.DirectoryServices Namespace.