John,
From the FSE manual on groups:
Example. Pilot fee is set to 85%. For a $10,000 assignment, the pilot will receive $8,500 and the group will receive $1,500. From this $1,500 the group must pay ground crew fees ($1,000), and aircraft rental. If the aircraft is owned by the group, then the group's profit will only be $500 (rental fee paid to the owner, which is the group: net effect is zero). If the group has to pay for aircraft rental, it is likely that $500 will not be enough to cover rental fees and the gorup will lose money on the flight.
The key note here is that the group pays for ALL expenses associated with the flight, INCLUDING booking and ground crew fees. Ground crew fees are generally 10%, then there are booking fees (the % is variable based on how many jobs you carry. The more jobs, the higher the percentage), as well as maintenance and fuel.
At 15%, the group could easily lose money on a flight before maintenance and fuel are even taken into account.