can we give them equal xp gains so that if you group with somone they wont out level you.
or maybe take the half exp 1 member would get for killing a creature, add that to one half of the exp the other member would get, divide it by two, subtract a certain penalty depending on how much higher the highest level char in the group is. then split the remaining exp
(((1/2 exp for char 1)+(1/2 exp for char 2)/2)- level difference penalty)
for each character, and you can expand it for more people.
im not sure what amount to use for the level difference penalty, but it should be a value that would make it more beneficial to have a group of players all the same level, or at least closer to one another levels. that way you wouldnt not have to out level a grouping partner, but if the level difference is to high then grouping loses some of its benefits.
or to do it the easy way, just split the amount of exp that the highest level character would get for killing a mob between the group.