I changed the time. Do people know?

Not on their own. Everyone who signed up before the change has the old time in their confirmation email and their calendar. Reminders do follow the new time, but the first one may be only a day before.

Steps

1. Change the date or time in chat, for example "change it to 9:30".
2. The builder's reply starts with how many people signed up for the old time.
3. Tap Email them the new time under the reply, on the website or in the app. Each of them gets the new time, what it was, and a fresh calendar link. Replies come to you.

Nothing is sent until you tap it, so you can fix a typo first. If you've reloaded the page, the button is still there until you use it. People who cancelled, are on the waitlist, or said they can't come aren't emailed.