you need to create a lookup recordset to see if the email address exists in the users table.
Set the Recordset to filter the email column on the email form element.
the recordset will create a variable named $numRows_<recordsetName> (where <recordsetName> is the name of the recordset) this can be used to determine if the recordset is empty, if it is empty, the email has not been registered.
to do this, go to Window -> Server behaviors and double click the Server Validation server behavior.
for validation type, select numeric. for the server variable enter:
set the minimum number to -1 and the maximum number to 0
this will cause the validation to pass only of the lookup recordset is emtpy.
you can use the validation sow if server behavior to show your email in use error message and link to the login page.