Something changed. Send an update and everyone who needs to know gets it.
Type what changed in your own words. It goes to the people who said yes and to anyone who has replied something — the people who have never answered at all are left alone, because a silent invitation should not become a stream of messages.
Change the date or the place and it becomes a re-ask. That goes further: everyone who said yes, everyone who said no, and everyone who replied anything. It says what moved and asks whether they can still make it.
The people who had said yes stay showing as yes on your page, marked as asked again, so your count never silently drops while you wait for them to confirm. You can see at a glance who has come back to you and who has not.
Updates go out straight away, except late at night — nothing we send lands between 10 pm and 8 am where the guest is.