Bounty isk - split between all fleet members who are on grid. So is the reputation hit (for killing faction ships that is). If you are member of fleet but not on grid (salvaging different room, sitting in a station or just flying through different region) you get nothing.
Turning in the mission - you can choose to either split the rewards, or get it all yourself. In first case isk, LP and rep are split evenly among all fleet members (including those in different region or even W-space), except faction standings (for storyline missions) - you always get these yourself. In second case you get the exact same amount of isk/rep as if you were doing it alone.
I suspect that if you have a friend who has higher standings with a certain agent, it would be wise if that friend took the mission, not you. This would increase your gains. In case when you don't have standings, but your friend does, but he doesn't have time to do missions which you could handle alone, he could take the mission, send you into site (you bookmark it) and just sit in station, then turn the mission in when you've done it.
Corp tax is deducted from each pilots share after splitting rewards between all pilots. If you and your friend are from different corps, you will get the same amount of isk, no matter what his corp tax is.