You should probably look at the 135 duty rules. They are written in blood. The worst thing about a 91 gig is that you can be flown into the ground, literally. Make sure you sit down with the owner and clearly state that there should be a 14 hour duty day limit. 8 hours of flying per day max. Also you need 10 hours of rest before starting the next duty period. Be sure to cover the scenario of rest starts when you get to the hotel, duty starts when the flight is scheduled not when they show up etc.

Now here is where you can negotiate effectively. Say look I want to make this situation work if you need me to break the duty rules it will cost you so much per hour on TOP of your salary. Anyone can understand that using an asset beyond the norm will cost. Granted the 5 partner situation will make this very difficult to manage because one partner may cause you to be taking a rest period and another partner will resent having to pay the "overage charge". However, you will save yourself from becoming burned out if your earning a lot of money. Keep in mind that money will not fix the fatigue situation for long. You can only be so tired while going 360 knots and not cause an accident.

I hope you understand what I am saying. Feel free to PM me about how a really long day can kill you. Trust me I have flown 91 and 135 and the only good thing about 135 is I can point to a rulebook when I am exhausted and should not fly. 91 your pretty well hosed. So you need to work it out ahead of time that your not a robot and you need to sleep.