New here and Need Help in “Natural SEO”!

I own and operate two sites for fun. If possible I may be able to get a business' to place ad's on my site. Like I said these sites are for fun not profit. I want to get more members though. How do I get my sites to pull in more members with search engines. Are there any tricks you could show an average person to make their site more visible. I have been able to figure out quite a bit from hands on experience as far as operating the sites goes. I thought it might be more than I could handle but sar far so good. Thanks ahead of time for any help you may be able to give.
Thanks again

if the sites contain images, optimize them for google images. its easier. after that, take a free link exchange script and start exchanging.

I'd go to similiar sites, and offer link exchanges with the site owner. What do you mean members? Is it an online community site? If it is, be prepared for lots of work, online community sites can be a lot of work.

http://webwisemedia.com/,
http://webwisemedia.com/

If you want to pull members from SEO first your site must be visible from SEO's. You can target a particular keywords and optimize your site on it. You can learn a lot from these forum on how to do that but it takes time.
I also suggest you a site called Affordable Search Engine Optimization, This site can help you a lot..
Goodluck and welcome!