Send email and in-app announcements to event guests. SMS is also available: use get_sms_announcement_audience, preview_sms_announcement, then send_sms_announcement after review. One message delivered by email (to guests with an address on file, minus unsubscribes) AND as an in-app push notification. Provide subject + message. Use when the host wants to tell their guests something (a time/venue change, a reminder, a thank-you). Outward-facing and can't be unsent — confirm the wording with the host first. Requires event_id; host only. Use for people ALREADY on this event's guest list; to invite new people use send_invitations; for your list/network or a newsletter use send_email_campaign.