How about give certain missions to certain players and increase the reward. If you group with that player then you share the mission and get the reward upon completing the match.

This way, bigger rewards are justified and missions promote cooperative play to achieve the same goals.

You could also just focus on promoting group dynamics through actual gameplay mechanics:
Squad Based bonuses
Squad-Functional Equipment (Nanohives & Uplinks)
Squad-Based Side Objectives
etc

If you want to really start making a difference in the amount of leaving here are some ideas:

1. Fix the match maker. Let people only queue against similar level players. Having new guys or lower MU players serve as "content" and cannon fodder for higher MU players is the most idiotic idea I have ever heard. If you know you are always going to go against similar SP pools and gun game each match then leaving will not make a difference. The very next game will be the same. It also means that high MU players have a reason to run high end gear...you will have to compete, but there will not be militia suits to pad your stats on, and leaving because you see strong players on the other team will have no incentive. Because if you are leaving for that reason you had better just leave the game all together, because every game will be like that. This will start to normalize all K/Ds as well. Which brings up the next issue.

2. Just stop tracking K/D. It means nothing in the game. Track ISK efficiency, WP/death, WP/ISK, or some metric that actually matters to a mercenary. Then guys that fret about their K/D have no reason to leave. It also means the more high end equipment you destroy in lower end gear the better your stats are. So another reason to stay. Besides there is no argument for any positive behavior that K/D promotes.

3. Pay out far less for losses or not at all. Works in FWs. Either you try to win or you lose ISK. We are Mercenaries and paying me to lose seems out of line.

4. Tax rate on assets over 100mill (or some other amount of assets). Major **** ups in the past have made certain players and corps way too much in assets. You need to add the risk and reward back to the game. Why not leave if you can't pad K/D if you don't need the ISK? After all, that's the only stat that's tracked. It is not the low Mu players leaving if you watch the boards, its the squads of stompers who actually meet resistance.

I am not sure if all four are the right answer or some of them or another idea all together, but leaving right now is pandemic. Certainly the first two would help. The third would either increase effort or just force more leaves late game. In the end this is just a game and people should be able to leave if they want. Its a recreational activity not a job or commitment, but it does need some changes to make it more enjoyable for everyone. The suggestions above are just my thoughts on the issue.