I think you need the UM API.
Have a look here:
Getting Started with User Management
If you require help on this specific topic, open a support case inside the Admin Console -> Support menu and ask for developer support on User Management API if the documentation is not sufficient.
EDIT: the UM API will not help on getting events on who is using or who is not using a subscription, rather who is assigned or not to a Product, add/remove/update accounts in Admin Console, get information about Org's groups and users more generally.