Actually, to my understanding, the Send email notification action doesn't rely on the user having an imported JSS account. Only that the device(s) in the Smart Group or from an advanced search, etc. has some user information synced to it that includes an email address. So you should be able to use that functionality.
Outside of this, there's always the old method once discussed here about using the unix mail command to have a policy run on the Macs in the Smart Group and send the email to themselves.
See here for what I'm talking about: https://jamfnation.jamfsoftware.com/featureRequest.html?id=611
EDIT: Sorry, I slightly misread your post. You are correct that the Email notification on group change function does require that the user has an account in the JSS and the option checked for their account. I thought you were referring to the manual "Action" method, i.e, pull up the group membership in the JSS, then click "Action" and choose the Send an email option. Screenshots attached.
Edit 2: I also didn't see you were talking about iOS devices and not Macs. My bad. The scripts referenced on the links above won't be much help since you can't make a script run on an iOS device. Your only option may be the Action > Send an Email process.
