Make sure you are advertising. Craigslist is a pretty good way to get the word out that you are *not* a private club and that you are a fun-focused group of people who happen to love horses.
Our riding clubs best turn out is for a poker ride. Anyone can participate and it benefits the club. You just need a mile or two of trail and a few decks of cards. We do a $10 buy in and split it 50/50. We also offer a second hand for $5. Most people do two hands. 50 riders, $750 total collected $375 for your club. Its fun, takes little to no skill and everyone has a blast. All you need is about 10 volunteers who can rotate taking turns riding the poker ride also. We usually have close to 200 riders.