you have described a very complex site, one that would rely heavily on databases and scripts, to say nothing of the financial transaction difficulties - it's relatively easy to automate payments in, but when you start on distributing money back to winning investors you are into deep water.

You'll also need to think very carefully about the legal implications. In the UK sweepstakes are classed as lotteries under the Gambling Act 2005, but where the sweepstake is restricted to a limited membership - such as in an office, or a private society of some kind it's OK. You've mentioned friends in America however, and there the situation is different - sweepstakes which require a payment in order to participate are prohibited under Federal law.

You'll need to check the implications for your American participants very carefully.

US gambling laws apply to American citizens, regardless of where the gambling site is hosted. No US citizen may enter a sweepstake if a payment is a condition of entry.

Conducting financial transactions via a website can be complex, and more so if you intend to make repayments to credit card accounts on an automated basis. That would require you to hold bank account details, and that in turn would require you to register with the information Commissioner as a data processor.

The fact that you would only offer the service to personal friends would keep you on the right side of the Lotteries Act, but be warned - if you started to open up to registrations from anyone else you will need a Lottery licence.

four member's comments about structure and security provisions are valid - this is not going to be a simple operation to set up.