That would be a good idea. Maybe having separate experience bars for every player you meet out in the world. The more you quested with this individual, the more experience you gained with that character. Each experience level could generate additional bonuses for when you play with this character. Bonuses would also stack the more you add to the group. Further, special dungeons could be unlocked for players that have an appropriate group unity level. That should promote a social aspect to the game.