An interesting side point to this is where do do players and replacement characters come in?
If I join your group, what level / XP total am I? Will I forever be a junior member unable to catch up, or am I coming in at the same level as everyone else? Do at come in at the lowest of the current players - which means my character is effectively penalized by the amount the person who attended the least is since we all get the same XP. Do I come in at the highest of the players and my XP losses atre only on my shoulders - but that means I've surpassed some people who have already lost XP.
How about replacement characters? If I die and come back with more XP, what does that say? If I die and come back with less than a new player, what does that mean?
These are questions for more traditional setups of one character per player, not ones with stables of characters for each player.