Is it illegal to use someone else's email to sign up or subscribe them to things?

Yes, particularly if you are using an email address which you found the passWord to without consent. And particularly even more if you sign them up for things that are spam, or require payment from the recipient. All of this constitutes computer fraud. If you misrepresent yourself as someone else with malicious intent (and as innocent as what you are doing may seem, it is malicious intent) it is computer fraud.

But you should still ask a lawyer if this is a legal issue of yours