For example, you hire 20 people. Each of them puts up $5,000. That's $100,000 right there.
15 of these people lose $500 their first week. Their "accounts" are now worth $4,500 each.
You took the opposite of their trades so you actually made $7,500.
5 of the people were profitable. Let's say they each made $500 their first week. Their "accounts" are now worth $5,500 each.
You took the opposite of their trades so you actually lost $2,500.
Your net profits at this point are $5,000.
Your firm has a 50/50 split so each of those people gets paid $250.
After you pay your people 50% of their $500 winnings, you have paid out $1,250.
Your profit for the week is $3,750.
Repeat. Encourage people to over trade so they lose more money. The more they lose, the more you make. When they blow their accounts they can deposit more money or you can fire them and hire new people.
In the event you get someone who is consistantly profitable (unlikely), just set it up behind the scenes so his trades actually go through and make money on his trades.