There is no "activation" with Exchange Server. Install it and it is in trial mode. Plug in the volume install key for Standard or Enterprise (I'm sure that's been leaked somewhere) and it is converted to that edition. User licenses are strictly on the honor system. Since you have no honor, just create however many you need. There is no enforcement. The above is true at least from 2007, possibly earlier, and continues through the latest CU of 2016. There's no updating needed for a new install, just get the latest CU and install from that.
Agreed 100% with what Threat said. Standard disclaimer that applies to most anything you read here: My posts are for informational purposes only, and confer no rights on the reader's part nor any obligation or liability on my part.
Because I assumed that the activation license depended on the number of users, I wasn't aware of the licensing until I read up on it a bit more and got feedback from other users.