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I could have sworn I saw a button that allowed you to send an email to all invitees to a particular event (I don’t mean the Reminders function). I don’t know whether it was native to Scoutbook, or part of Feature Assistant, or a figment of my imagination.
The message invitee was (I think) primarily intended to message based on RSVP status. If RSVPs aren’t on, it’s hard to tell if they were never on or just aren’t on now. I suppose there could still be a desire to message all of the invitees (separately from their RSVP status), but I tend to only do that if I have RSVPs on.