Computers count as a device when it comes to a six-device limit. From Netflix's FAQ:
"Please keep in mind that your account can have up to six unique authorized devices activated (associated with the account) at any given time, including personal computers and Netflix ready devices."
Registration is automatic; odds are good that most people aren't even aware that this is (and has been) happening. Interestingly, as best as I can tell, Netflix's license agreement does not prohibit your sharing your password with friends. However, you are limited to six devices, including computers, and up to four simultaneous streams, depending upon your plan.
As for whether the disc is DRM, it seems unlikely. Who needs a physical key when you have an account and a device ID that is stored on the server?
"Confound these ancestors.... They've stolen our best ideas!" - Ben Jonson