I'm going to be starting a longboard club and wondering if anyone has any tips on how to get people to know about us and to get the community more involved, and how to go about getting city's approval to use a park for an event.
I doubt it will be very easy for you to do something at a park. I know in NYC, you need to apply for a permit for the event through Parks & Recreations, then depending on the size of the event, they may need to contact your local police department for security. On top of that, city permits aren't cheap either, if I'm not mistaken, they're about $300 and up. That's here in NYC, I'm not sure if it's the same for your area.
Post flyers and go to local colleges. Post all the details about the club (like if you'll be providing boards or other gear [i doubt you will, thats extremely pricey]). And about the park, go to your local city council or something and check with them.