Hi Carolyn , if the event was posted prior the user joining the forum, then they won't receive a notification of the event. Unfortunately there isn't a way to re-trigger it, however one option is to add a link to the event in your Welcome Email that is sent to users when they join.
This is configured in Admin > Settings > Welcome Email. Users receive this email once when they first join the community. You can add any important information they should know when joining the community, including adding links to upcoming events or other community areas. You can edit this email any time to keep it current.
For users who already RSVP'd or Followed the event, they will receive an email notification if the event is canceled or rescheduled, or if a new reply is added to the event.