Hi Kate,
As Iñaki mentions, an admin can create accounts with the same email address.
Thus, you have two options (in standard Moodle) in Site administration > Users > Accounts - Add a new user and Upload users.
Thanks for highlighting the fact that it is NOT possible for new users to sign up for an account with an email address which is the same as another account. I have added a note of this to the documentation Managing authentication.