The Purpl PRO is operated using the Purpl App. To use the Purpl App, you must register a user account. All that is required is an email address, and a password that you create. Once you've created a user account, you can register a device to that user account. In other words, the device is tied to the user account, which is tied to an email address.
The email address is used to identify you to our systems, but is also necessary if you need to reset your email. Verification emails are sent to this address to. This is the only way we use your email address (it is not sold to or shared with anyone).
If you share your device with another person (for example in a work setting, or simply because it's a free country) you must also share your login information, because your Purpl PRO is tied to your specific user account. This means your also sharing your saved measurement history within the app and the web-based online Portal (https://webportal.purplscientific.com/login).
Alternately, you can have the person that you share the Purpl PRO with set up their own user account. In this case, the device must be deregistered from the original account (yours), and re-registered with the new account (the person you're sharing with). This is done within the "My Devices" screen in the app. When the device is returned to you, the device must first be de-registered from the account you shared with, and then re-registered to your own. This can cause issues if you no longer have access to the "new" account.
Ultimately, the plan is to add a level of user accounts so that a user that owns a device can create sub-accounts to share the device with friends and coworkers.