First, let me say that as far as WordPress goes, no – it’s not possible.
But it is possible with some processing that goes around the WP process. However, it is not recommended (and also not possible within the plugin natively), and it may or may not be compatible with other plugins when used.
With that said, in the past I have provided support site subscribers a custom process in the form of a code snippet for this and also included that in the Advanced Options extension.
The process essentially spoofs WP into thinking a unique email is being used and then after the user is added, it updates the database directly changing the email from the spoof/fake to the actual email used.
It’s not a perfect process. I don’t have a way to handle updating the user profile on the admin side as the email does get flagged as not unique. But other than that, it does work.
