Is there an easy way in Plone to get email notifications when new users join the portal?
If you want to do it the right way, follow Martijn's directions above.  If you want another dirty solution, do the following:

make a copy of register.cpy
insert the following code:
context.MailHost.send('Content-Type: text/plain\n\n' + 'User has registered: ' + str(username),
            mfrom='me@gmail.com',
            mto='you@gmail.com',
            subject='User Registration',
        )
I want to get email notifications to the portal email address whenever a new user joins the portal.
My guess is that I should code a new product to do that.
Does such product already exist (for Plone 4)?
I checked content rules, but AFAICT it could work only if I made the users contentish themselves with something like membrane/remember, but for my requirements that would be overkill.