When you purchase a copy of Virtual Drumline, you are actually purchasing a license to use the sounds and software. Technically you do not "own" the software. You own the license to use it and make recordings of your compositions created with the software.
Per the included licensing agreement, Virtual Drumline is a single-user license. This means you are permitted to install the software on two of your own computers. You may not share this license with other individuals. Each individual user must have his or her own serial number to own a license.
Software authorization for all Native Instruments products is handled through their registration system which tracks your serial number(s) and hardware profiles, assigning them to your NI user account through the Native Access application.