This is a serious problem with your software. It appears you are not giving it serious efforts to fix since it has been on the list since at least May. All you need to do is take a look at the roster right before the invitation and reminders are sent. What you are doing now is taking a snapshot of those in the selected list when the event is created and not looking again when it is sent. ALL troops do their planning and event creation before the roster is complete. This cannot continue. It is fairly simple code. You should have been able to take care of it months ago. Just do a lookup before sending the message, or creating a signup sheet, etc.
What you are doing is requiring each and every event to be opened and re-saved every time another person is added to the roster.