TradeHill now has a referral program where you can get others to sign up using your code and earn a commission each time. Here's the details from their site:
"Participating in the TradeHill Affiliate Program allows you to generate referral codes. Anyone who creates an account with TradeHill using your code will generate revenue for you when they trade. This code can be placed in ads, used on forums, sent in emails and a variety of other ways. Funds earned through this program will automatically be credited to your account. You can find your referral code by going to your user profile. This code can be provided to other users or can be embedded in URLs [http://www.tradehill.com/?r=YOURCODE]. If you're interested in learning more please send an email to Info@tradehill.com
or contact us."
That being said, if you want to join up on tradehill, here's a link to get started (and referring others!):https://www.tradehill.com/Support/ReferralProgram