We bill most of our customers on a per-machine basis, but we have a handful (mostly RDS / Terminal Server environments) that we bill by the user. N-Central has access to the information via the Domain User Management section, but I don't believe it is available via reports anywhere.
We're not looking to build an audit trail with this data, but simply need a snapshot to adjust monthly billing. For our purposes at least, we don't care that 5 new users were hired and 4 others were let go, we simply need to know to bill them for adding 1 employee since last month.
I think the simplest solution for us would be to add an Export -> CSV button to the top of the Domain User Management page similar to the one at the top of All Devices view.