It may not be possible in your area but we got lucky with a court in a high visibility area (ours is a centrally located park near muni metro line) LOTS of people walking by and watching- We always try to invite them in to try it. There are lots of bikers here in SF, most will stop by with totally non-appropriate polo bikes (long gear fixies with drop bars). We invite them in and loan them our bikes, mallets and encouragement. Most never come back, you just need to work the numbers.
I'll re-iterate what Alex said, more than one night per week adds to the visibility and enables you to have a 'beginners night'. Tweeting or FB'ing the fact that you're having a newbie night will encourage more folks to come out for their first time. The most common comment I hear from folks walking by is that they're intimidated by a couple of things- "I don't want to get hurt", "My bike handling skills are not as good as y'alls" "I don't have the right bike for it" These folks all want to play but are talking themselves out of it because they see you pulling off the insane moves and shots- If they see a couple of games with people playing who can barely control their bikes and whiff at every shot they take, they might think "Hey I can do it better than that!" and jump in.
Try to get your group to do a demo at local college campuses. Hit their bike racks while you're there with spoke cards advertising newbie night next Saturday night.
And last but not least, bring beer. Having a beer to offer someone while they're watching works wonders! Then you can say "No biggy! just pay me back when you come next week with your bike!"
good luck!
I think its important to keep trying to involve new people, because as players get more busy/ get injured/ move, you need new blood. For this reason, i think beginner games are very effective. It lets people try out the sport in a slower paced, more relaxed environment. It also allows them to get better because they are competing with people closer to their own skill. Heres a few other suggestions:
-If you can, bring a bike (or 2) to loan out to people who dont have a dedicated polo bike, or have one that sucks. Low gear is important.
-Travel as a group to the nearest polo playing city. This will introduce your players to different playing styles.
-Try and set up more than one night a week for polo. One week night and one weekend day will give people with different work schedules more chances to show up.
-If you can spare it, give out mallet materials, disc material and bike parts to those who need it. Alot of times someone wont show up because they just need somebody to help true their wheel, or they need a brake cable.
-I barely understand Twitter, but it seems like a quick, easy way to communicate to alot of people. We just set one up for polo in our city, and its alot more efficient than trying to send out and then respond to 20 text messages every polo night. Try it out. Youll be more likely to get more people to show up, and more consistently.
-Try throwing your own small tournament. If you dont have enough people, make it 2 on 2 games.