What if you had a status bar above the players with a bar that shows how hostile they have been to you / you have been to them. Green for friend and red for enemy, and the degrees inbetween.
If you shot a person he get redder, if he shoots you you get redder. If you gives him a ride he gets greener, if youre close without fighting he gets greener.
This could be a client matter based on nick, and mabye stored localy inbetween games. Since there are a lot of people playing it could be fun to see if a person has been very hostile against you, and who it would be fun to team up with.
You should also be able to add persons to a friend and enemy list manualy, since there is always someone who will always be a friend no matter how many times he shoots you. (Gangs and real life friends).