Advertise your event in the local community newspaper, there is usually a free community events section
Use Local Community websites or social pages (Facebook has lots of community pages)
MAKE A SIGN. Put up a 'Coming Events' sign and make sure it welcomes non riders (the general public)
Invite local Board or Council members or politicians!!
Make sure there is a sign (or a volunteer) at the entrance telling non-horse people they are welcome to attend, where they can park, and what they might see (how to stay safe).
Offer guides\pamphlets to events