Using an administrator email still allows signup to Memberful

[lukesiedle]

After some confusing debugging I realised I was using an email address associated to my superadmin (while being logged out of admin), and the after signup login wasn't working, but the account was still being created in Memberful.

Steps to reproduce:

1. Create an admin account in WP
2. In a separate browser window/incognito, sign up with a Memberful account using that email address
3. Expect to be logged in, but state does not change
4. Attempt to log in with this email address/password
5. This takes you to an admin login page

Maybe there is some way you can tell the user this email is in use without creating an exploitable scenario.