Don't allow scheduled reminders for events to also include groups
Edit: See discussion below, this doesn't actually work anyways.
If a user is able to edit events and has access to a particular group, they can add that group to a scheduled reminder for an event with Also include. It doesn't seem like a good idea to let users scheduled a reminder to potentially a very large number of contacts via an event scheduled reminder, which is intended to email participants for an event.
I can see the case for also including manually selected contacts, but also including groups seems dangerous and an invitation for mistakes. If a user needs to send a scheduled reminder to a group, they would probably be better off using a mailing or a separate scheduled reminder.
Proposal: Disable the Also include > Select Group option in scheduled reminders for events.