You don’t need to be logged in for the app to work.  There are some functions that will not be available, can’t remember what though.

Log in with what email? Yours or somebody else’s?

If you’ve forgotten your password, request a new one on the Sonos website.

If somebody else’s email address is populated under the account, and you want the system to be registered to you, use the Transfer System Ownership option in the app and follow the prompts.

You may want to refer to the manage your account FAQ.