To side step the issues associated with manging money for multiple people (like licensing and Fed/state paperwork), you can create an LLC or LLP. Then have each person "invest" their cash/assets in the LLC/LLP and thus have a representative share of the company. You can then trade the LLC/LLP's single account and the profit/loss would distribute proportionately to the investors.
If you're going to take a management fee, you'll probably want to make the primary vehicle an LLP and create a separate corp (C, S, or LLC) just for yourself. Then make that corp the general partner of the LLP and charge it with management and extract a management fee from the LLP for it. This makes it simple to establish a management contract that gives you a % of the profits realized if that's part of your plan.
Note however that to keep everything legit, you'll need to manage the LLP (and the second corp) properly (i.e., board meetings, minutes, proper state filings, tax returns, etc.).
If you're not familiar with what this stuff, start out by going to Borders or Barnes & Noble and reviewing the various books on incorporation that you'll find in the Business section.