After the meeting, the list is the record
Reminders do their work in the background: emails at twenty-four hours, an hour and fifteen minutes before the start, to everyone registered unless you switch them off. There is no WhatsApp nudge sent to staff on your behalf, and no sync into anyone's calendar โ the confirmation email carries a calendar file and a Google Calendar link, and that is the extent of it.
Once the session closes, export the list and send one follow-up to a chosen segment of the registrants with the deck or the recording link you are supplying yourself. For the departments you expected and did not see, the honest route is the manager who owns that team, not a message the product cannot send to people who never registered.