Just did some testing to see how experience is affected when in a party. It seems like when you are in a group, exp per kill is reduced by a percentage depending on how many group members you have:
1 member - 100% (normal) exp
17 mobs for 1% normal exp
2 members ~ 85% normal exp / kill
20 mobs for 1% normal exp
3 members ~ 75% normal exp / kill
23 mobs for 1% normal exp
did not test 4 members, one would assume 4 members is something like 65% normal exp / kill.
Contrary to popular belief, it DOES NOT divide the experience between members. As long as you are killing faster in a group, you should gain more experience and loot in a party.
Please feel free to contribute any information and/or testing you have conducted as a reply to this thread. Thanks!