Entity Reference - Not required to be in database
Entity Reference can reference entities only. It would be a really big hack to the module to allow non-entities to be referenced. But there is no need to.

There are dedicated modules for exactly what you are trying to do. Instead of Entity Reference, create User Relationships. Use User Relationship Invites submodule, it


  allows users to specify a relationship to a user that they invite to join the site.


Invitation is done by using e-mail, so it seems this will fully satisfy your use case.