My wife and I have been sharing an account and two Readers since the PSR-500 came out three+ years ago. It works fine.

To manage devices:

- Launch Reader app on PC
- click on "eBook Store" on left-hand side
- log into you account by clicking onto "My Account"
- click on "Manage/Rename Devices"

I don't recall for sure, but I suspect if you at this point plug in an unknown reader, it will ask you if you want to authorize it.