Experience mechanics are fully explained on the wiki page:
http://wiki.project1999.com/Experience
I don't think it matters, though. If you're a level 30 grouping with a level 40, they are carrying you (hopefully), so they get a larger share of the exp.
As for exp "penalties," they are better described as "appetites." And both racial and class exp penalties are distributed across the group. That does NOT mean you get a 68% penalty if a Troll SK joins your group. That just means the Troll eats more. Just read the wiki article and come back with any questions you may have.