"Experience sharing" works as you would expect it, regarding attacks: The one who deals more damage, gets more experience.
In which way the received damage is shared, is yet unknown to me; sometimes it seems that one of the group members is preferred by the opponent, maybe the first attacker. This can give an advantage to unequal groups: One who is very well trained, especially in armor, might start a defensive attack, attract all damage (which blows off on the armor or agility), and the weaker members can attack rather safely meanwhile.
I hope that moderators will not scold me for this suggestion too much, because this behaviour supports cooperation between different members (previously I used to assume that only similar characters should group).