You must permit an array [] in your controllers params method.
In the user_events_controller.rb I now have
def user_event_params
params.require(:user_event).permit(invited: [])
end
You then may get an error as your database probably won't allow you to save an array to it.
This can be fixed my adding serialize :invited
to your model, in my case user_event.rb. This is cool because when you retrieve the attribute from the db in turns it from a string back to an array.