You would need t create a recordset to filter your existing registered users table on the email address entered in the registration.
You could then use the server Validation in the CSS Form Builder extension to make sure that the recordset is not empty.
After installing the CSS Form Builder extension, you can double click the Server Validation server behavior on the server behaviors panel to edit it.
Slect Number validation, for the server variabel use "$totalRows_recordsetName" (Substitute the name of your recordset for recordsetName).
Set the minimum to 1, leave the maximum blank.
This will cause the registration to be precessed only if the users email address exists in the existing registered users table.