When you first create your character it asks you if you will be sharing the same internet connection as another player. If it is true and you put their handle in the box, does this mean that you have registered with the email@example.com? Or do you still have to do that to? What is the difference if any?
If you enter someone's handle, it will not automatically register you; it displays the other's handle in a separate field for the people doing join requests. Usually we manually add you as registered multis as we accept the account, and you can check your multi stats at the bottom of the Account Settings page.
If, after having the account approved, you still do not display as registered multis, you should e-mail firstname.lastname@example.org. However, if you are already registered as multis, there is no need.
(On a side note, this may not be working properly for everyone at the moment, as I am a registered multi and it is not displaying properly. I've got an open bug report about it, so hopefully it will be fixed soon.)