Umm, I am not sure I understand. Your roommate can join your system by adding his controller to it, but if the system is now part of your account, all of your roomate’s data will be gone.

Any Sonos system can have up to 32 associated controllers, and they don’t need to have the password, that’s used for system level changes, such as adding streaming accounts. 

Nor am I sure why you felt the need to repost a quote of your original post.

Your question is not clear, @Serrano house. If you set up a new account and added his speakers to your account, Sonos will assume you’ve become the new owner of those speakers. Because they are now part of your system, they get removed from his account, so of course he can no longer access them from his account.

Is that what you’re asking/what you’ve done? If so, you need to reverse the process to “give them back” to him. 

You cannot use the same speakers on different accounts. You both need to log in in the app (if necessary) using the same account.