How do you invite a facebook group to a Zwift meetup?
You cant really, you have to invite them all individually. However, if you are in a club you can share club events with a link and they can join from there (if the club event is configured to allow non club members to join).
If you are a rider above level 20 or a runner above level 10, use the Zwift companion app to create a Club. Then you can create events for your club, producing a link you can share in your FB group. Anyone with the link can then join then the event. Hope this helps.