This worked for me. And yes, the solution is a bit annoying. I changed my Apple ID, and email address because we went with a new provider. I lost all ability to add any alerts at all to the events I had set up. I used them to send reference information to people at an appointed time.
Make sure you add your new email address to Contacts on your own card. Don't edit the old one. Delete it and add your new one. Quit Contacts.
Quit Calendar and re-open it.
You'll have to delete the previous event you had created and create a new one. You should now have the option to add reminders including custom reminders, or emails. (if you put your list of people into Contacts under your card.)
It's a pain in the face to have to do, but it does fix it. I had one that went out to different people each week for the rest of the year. Now I have to spend time restoring them. Pooh.
~Deb