You can send an email using
MailMessage[
^] containing all the information that you want to display to the registered user.
Instead, what you can do is, once you register any user, it means that you're saving his/her info to some database.
So fetch the info that is necessary for login i.e Email/Password and show it through the
Modal Popup[
^].
I hope the idea is pretty much clear to you :)
-KR