There is no way of doing it on Mir 3 (That I am aware of), there are loads of options for adjusting the REINCARNATION and Master/App %'s, maybe one of those will adjust Group's (Doubt it though
). We have disabled Master/App on Sting to encourage more group hunts, maybe the way to go?
Its also true that you will always level quicker solo than in a group (Unless your a noob), group's should be required to level in harder places, kill harder things, or simply last longer.
P.S. Excellent Web Site BTW