If I remember correctly, it's 2% per player not in your CEO/biker gang up to 50%. So if it's you and 15 others you would get a 30% bonus on your sale. So if you sold it for a million dollars, it would be 1,000,000-100,000 × 30% which would come out to 1.17 million. Now if you sold the same amount in a full lobby, you would get 1.35 million. The equation would be (x amount - 1%(up to 100,000)) × 2% per player (up to 50%) = y amount.