1zaak not possible out of the box. Flarum requires account to have a unique email. That's where notifications are sent, how social login can tell users apart and that's how passwords can be reset.
You could develop an extension that will remove the email field, but it will likely have to replace many components of Flarum that expect a valid email.
Nothing prevents users from using throwaway emails, or you manually assigning emails from a domain you own.